GCSE Science podcasts - Bonding, structure and properties

TULELA: I’m Tulela Pea, a science communicator and podcaster.

SUNAYANA: And I’m Dr Sunayana Bhargava, scientist and poet.

TULELA: And this is Bitesize Chemistry. Hello lovely podcast listening friends. We’re back with all things GCSE Chemistry and combined science. Good to see you again, Sunayana.

SUNAYANA: You too, my friend – especially seeing as we seem to be getting on so well in series one.

TULELA: Yes, we definitely bonded.

SUNAYANA: Is that a clue as to what this series is all about?

TULELA: Yep, you got it! Together, over the next eight episodes, we’ll be looking at the key concepts of bonding, structure and properties of molecules and compounds in chemistry.

SUNAYANA: With some real-world examples, some analogies to help us understand the ideas in different ways and we’ll round off each episode with a quick quiz and the key facts to remember.

TULELA: Can’t wait, Sunayana. Let’s do it.

SUNAYANA: In this series, we’ll be focussing on different ways that atoms are bonded together and how the structure and properties of these compounds differ, depending on the way in which they are bonded. So, let’s look at why bonding is so important in chemistry.

TULELA: Well, simply without bonding, there is no chemistry.

SUNAYANA: So, you’re saying no molecules or compounds, which means no water, no carbon dioxide - which almost certainly means no life.

TULELA: Sure, but it’s worse than that, we’d have no stable structure at all. The universe without chemical bonding would be boring and lifeless forever.

SUNAYANA: Let’s get some background. We’re joined again NNICK. NNICK, can you give us some background on the different types of chemical bonding?

NNICK: Bonds. Chemical bonds. [EMOTIONAL MUSIC] Is there any stronger bond than that between a she-wolf and her pup? Or a she-moose and her calf? Or a she-hamster and her… calf? Yes. There are at least three stronger bonds, actually: ionic bonding, covalent bonding and metallic bonding.

Ionic bonds form in compounds between a metal and a non-metal. Electrons are transferred from the metal atom, which becomes a positive ion, to the non-metal atom, which becomes a negative ion. The ions are strongly attracted to one another. But it’s an electrostatic attraction – no funny business. Ionic bonds are very, very, very, very, very, very, very, very strong.

SUNAYANA: Thanks, NNICK. Let’s unpack that a bit.

TULELA: Ionic means that the atoms involved in these compounds become ions. And the ionic bond is the force between those two oppositely charged ions to form a compound – it’s called an electrostatic force – so that each of the atoms in the compound has that stable full outer shell of electrons.

SUNAYANA: This ionic bonding occurs between metals such as those found in groups 1 and 2 of the periodic table and non-metals.

TULELA: Group 1 elements have one electron in their outer shell which they want to get rid of so they become positively charged 1+ ions when they lose that negatively charged electron. Group 2 elements have two electrons in their outer shell and so become 2+ ions when they lose their outer shell electrons. Positively charged ions are called cations.

SUNAYANA: A good way to think about how losing something - in this case, electrons - can make it more positive, is if you have a cold bug, which is a negative thing to have and then imagine you lose that bug and feel better, feel more positive.

For example, sodium in group 1 loses an electron to become sodium plus ion. Magnesium in group 2 loses two electrons to become magnesium plus 2 ion.

TULELA: The non-metals involved in ionic bonding are in groups 6 and 7. The group 6 elements are two electrons short of a full shell and so they want to receive extra electrons to complete their shell, and group 7 are one short.

SUNAYANA: So, when those atoms receive electrons to complete a full outer shell, they become negative ions. So, for example, oxygen in group 6 gains two electrons to become oxide 2 minus ion and chlorine in group 7 gains one electron to become chloride 1 minus ion. Negatively charged ions are called anions.

TULELA: Sunayana, we keep talking about atoms wanting to lose or wanting to gain electrons because they want to have a complete outer shell. But they don’t really want anything? Atoms don’t think anything.

SUNAYANA: Of course not. I suppose this is just a convenient way that we humans like to think about what’s going on. In reality, atoms are simply much more stable in compounds when they have a full outer shell. It’s like saying that a chair is more stable when it has four legs rather than three legs. We wouldn’t say that the chair wants an extra leg. But saying that an atom wants to lose an electron is just a nice way that we can imagine it.

SUNAYANA: Tulela, fancy some ionic bond examples to try out?

TULELA: Go for it.

SUNAYANA: Let’s start with a nice easy one – our good old friend table salt, sodium chloride or NaCl.

TULELA: Ok, sodium is in group 1 so has one electron in its outer shell. Chlorine is in group 7, so wants – or is more stable with - an electron to complete its outer shell. Sodium gives up its electron and becomes sodium plus ion. Chlorine receives that electron and becomes chloride minus ion. So an electron is transferred from sodium to chlorine - and since oppositely charged ions are attracted to each other through the electrostatic force, they become bonded. The ionic bond. How about magnesium chloride?

SUNAYANA: OK, magnesium is in group 2 so has two outer shell electrons. Chlorine we’ve already said is in group 7, so wants just one of those electrons. So, in this case one magnesium atom donates its two electrons to become magnesium 2 plus ion. And two chlorine atoms take one each of those electrons and both become chloride minus ions. So, the compound is Mg-Cl2 which means one magnesium atom and two chlorines. And again, the electrostatic attractions between the magnesium ion and the two chloride ions are the ionic bonds. Final example – sodium oxide.

TULELA: We’ll go through this one in a moment, but if you want to have a go yourself, press pause now whilst we look at some cat pictures on social media.

SUNAYANA: Ahhh… look at that one…

TULELA: Sweet, right?

OK, we’re back. So, sodium oxide: sodium, group 1, one outer shell electron. Oxygen, group 6, needs two outer shell electrons. So, in this case two sodium atoms each give up their electron, each becoming sodium – or Na plus – ion. The oxygen picks up the two electrons and becomes oxide or O 2 minus, and the compound is therefore Na2-O.

SUNAYANA: It can be useful to draw a diagram to describe what’s going on in these ionic bond formations, to show the outer shell arrangement of electrons in the metals and non-metals before and after the transfer of electrons. You can find some useful diagrams of these on the Bitesize website.

TULELA: You may have seen dot and cross diagrams where we draw dots to represent the outer shell electrons of one ion and crosses for outer shell electrons of the other. Doesn’t matter which, as long as its dots for one and crosses for the other. So, for example, we’d draw one dot for sodium’s only outer shell electron, or two for magnesium’s outer shell electrons.

SUNAYANA: And the electrons in the non-metal outer shell we’d then represent by crosses. So 6 for oxygen and 7 for chlorine. Again, we’re just interested in what’s happening with outer shell electrons so we’ll only draw them, rather than all the electrons in all the shells.

TULELA: We draw arrows showing that the dots – the metal electrons - are transferred to the outer shell of the non-metal to join with the crosses. In an exam, you need to draw square brackets around the ions that are formed – you can find examples of these on the Bitesize website.

SUNAYANA: Time for a quick Summary, Tulela?

TULELA: Start us off.

SUNAYANA: Bonding is important in chemistry, as that’s the process where molecules and compounds form.

TULELA: Ionic bonding takes place in compounds made from metals and non-metals so that the outer shells are completed in the atoms.

SUNAYANA: The metals lose electrons to become positively charged ions.

TULELA: And the non-metals gain electrons to become negatively charged ions.

SUNAYANA: Dot and cross diagrams are a useful way to represent this.

TULELA: The next episode is more on ionic compounds, looking at their structure and properties.

SUNAYANA: And don’t worry as I’ll be here too - don’t want to break that bond now do we, Tulela?

TULELA: See ya.

SUNAYANA: See ya.

SUNAYANA: I’m Dr Sunayana Bhargava, a scientist and poet.

TULELA: And I’m Tulela Pea, a science communicator and podcaster.

SUNAYANA: And this is Bitesize Chemistry. This is the second episode in an eight-part series on bonding, structure and properties. In this episode, we’re going to look at properties of ionic compounds and how we can represent the structure of ionic compounds with different diagrams and the advantages and disadvantages of each.

TULELA: Remember to have a pen and paper handy to take notes and draw diagrams along the way. Let’s do it.

SUNAYANA: On the previous episode, we looked at how ionic bonding occurs between metals and non-metals to form ionic compounds.

TULELA: Quick recap, the metals in group 1 or group 2 of the periodic table donate their outer shell electrons to non-metals in groups 6 or 7. The metals become positively charged ions and the non-metals negatively charged ions.

SUNAYANA: And the electrostatic force of attraction between these oppositely charged ions is ionic bonding.

TULELA: And we looked at how we could represent this in a dot and cross diagram showing the transfer of the outer shell electron from the metal to the non-metal.

SUNAYANA: OK. Let’s now look at why that dot and cross diagram doesn’t give us the full story. It only shows the bonds and transfer of electrons between two or three ions and in two dimensions. In the real 3D world, ionic compounds are formed in giant lattices built of lots of ions where the forces act in all directions of the lattice.

TULELA: If you imagine, for example, our old favourite table salt sodium chloride – NaCl – just one grain may contain about one quintillion sodium and chloride ions. That’s one with 18 zeros after it. Each sodium ion in the lattice is bonded to a chloride ion above, below, to its left, to its right, behind and in front. And the same is true for each chloride ion – they're bonded to a sodium ion in all directions. You can think of this lattice as a regular repeating ordered pattern of those ions.

SUNAYANA: You’d need a pretty large sheet of paper to draw one quintillion dot and cross diagrams in a three-dimensional lattice.

TULELA: You’re right about that. It can be tricky to draw exactly what an ionic compound looks like, but we can use different models to help us.

SUNAYANA: Models in science are a convenient way for us to imagine sometimes complex ideas and each model has its advantages and disadvantages, but taken together they aid our understanding. So, let’s have a look at what models we can use to describe ionic compounds.

TULELA: We’ve looked at the dot and crosses diagrams already and seen that they’re good for showing how electrons transfer from the metal atom to the non-metals. However, they don’t tell us anything about the size of the atoms nor how they are arranged in the lattice.

SUNAYANA: We can also represent the compound in two dimensions by showing what atoms are in the compound and how the ions of these atoms are connected. And also to give an idea of the relative size of the ions.

TULELA: Let’s take the example of sodium chloride. Grab that pen and paper and draw along. If you have a couple of different size coins, say a 10p and a 5p, first draw around the 10 pence coin.

SUNAYANA: Representing the chloride ion.

TULELA: On a blank page and then next to it draw a five pence coin.

SUNAYANA: Representing the smaller sodium ion.

TULELA: Make sure that the two circles you’ve drawn touch each other but don’t overlap, and alternate these across the sheet. Then, on the next row down, start with the 5p and alternate again with the 10p and so on. Eventually, you’ll build up a two-dimension space filling model of the lattice.

SUNAYANA: Pretty useful as it shows how the ions are arranged in one layer of the lattice. But it doesn’t show how all the other layers fit together.

TULELA: For that, we need to venture into – de-de-de-de de-de-de-de – the third dimension.

SUNAYANA: It’s not that mysterious, Tulela. A 3D space filling model is a bit trickier to imagine, but there are some great diagrams on the BBC Bitesize chemistry webpages.

TULELA: We can also make 3-dimensional structures using balls and sticks, where the balls represent the ions and the sticks represent the bonds. We can draw these on paper but again this has a limitation of trying to represent something in 3D on a flat 2D page – or, we can create them in our wonderful 3D world using plastic model kits or computer models.

SUNAYANA: In each case, they are helpful to visualise the structure as we can rotate them to look around the lattice and get a much better idea of the shape of the ionic compound.

TULELA: But they are also misleading as the sticks make it look as though there’s a lot of space between the ions when, in reality, there isn’t. The forces between the ions are in all directions, not just along the length of the sticks keeping the balls together.

SUNAYANA: So, there are various models to help us visualise the structure of that giant ionic compound lattice.

TULELA: In which strong electrostatic forces of attraction act in all directions between the oppositely charged ions.

SUNAYANA: And it’s that very structure and the bonding that helps to explain the physical properties of ionic compounds.

TULELA: If we look at melting and boiling points first. Since those electrostatic forces between the ions are strong, it takes a lot of energy to overcome them, and so ionic compounds have high melting and boiling points.

SUNAYANA: For example, sodium chloride melts at just over 800 degrees Celsius and boils at around 1400 degrees Celsius.

TULELA: When it’s a solid, the ions in the lattice are held together by that same strong force, and so the solid compound cannot conduct electricity, since electricity is simply the flow of charged particles from one place to another. Ions can’t flow when it’s a solid.

SUNAYANA: However, if we melt the solid, or dissolve it in water, in this state as a liquid those ions separate and are all free to move. So in this state, an ionic compound can conduct electricity as charge can now flow.

TULELA: You know Sunayana, we haven’t heard from our AI chatbot NNICK this episode. I think we need to give him something to do… Hi NNICK, can you give us a quick multiple-choice quiz based on the structure of ionic compound lattices? And do write down your answers, dear podcast listening friends, as we go along.

NNICK: It’s the ionic bonding multiple choice quiz. Question 1: Are ionic bonds held together by: a) glue b) string c) hopes, dreams and wishful thinking d) electrostatic forces, or e) intermolecular forces.

TULELA: That would be electrostatic forces.

NNICK: Question 2: The dot and cross ionic bonding diagram is limited because: a) it gives no information about bonding b) it does not show the 3D shape of the molecule or c) Yes.

SUNAYANA: Answer B – it doesn’t show the 3D shape of the compound.

NNICK: Question 3: True or false? When ionic compounds are dissolved in water they cannot conduct electricity. a) true b) false c) none of the above. Hmm, that's a tricky one.

SUNAYANA: False – ionic compounds can and do conduct electricity in water – and there’ll be more on this in series 3.

NNICK: That’s the end of the quiz.

SUNAYANA: Thanks, NNICK. Quick summary of ionic compound structures, Tulela?

TULELA: Indeed. Ionic compounds have a giant ionic lattice structure.

SUNAYANA: The ions are held together by strong electrostatic forces of attraction.

TULELA: We can visualise the structure in 2-dimensional drawings as well as 3-dimensional models. Each model has advantages and disadvantages.

SUNAYANA: And the structure and bonding in ionic compounds explains their high melting points and whether it conducts electricity if solid or liquid.

TULELA: I’m Tulela Pea.

SUNAYANA: And I’m Dr Sunayana Bhargava.

TULELA: And this is Bitesize Chemistry. To hear more, search Bitesize Chemistry on BBC Sounds. Thanks for listening.

SUNAYANA: Bye.

TULELA: I’m Tulela Pea, a science communicator and podcaster.

SUNAYANA: And I’m Dr Sunayana Bhargava, scientist and poet.

TULELA: And this is Bitesize Chemistry. This is the third episode in an eight-part series on bonding, structure and properties. In this episode, we’re going to look at covalent bonding and molecular compounds.

SUNAYANA: And we’ll look at how those covalent bonds affect the properties of molecules and compounds.

TULELA: Remember to have a pen and paper handy to take notes and draw diagrams along the way. Let’s do it.

Time for a quick overview of covalent bonds before we get into the detail. NNICK our AI friend is here. NNICK, can you give us a summary of covalent bonding, please?

NNICK: A covalent bond forms when two atoms share a pair of electrons. The bond is very, very, very, very, very, very, very strong. Covalent bonding occurs in most non-metallic elements and in compounds of non-metals, often forming substances made up of small molecules. While the bond between atoms in these molecules is very, very, very, very, very, very, very, very strong, the bonds between the molecules are, in contrast, not very, very, very, very, very, very, very, very, very strong. The substances therefore have relatively low melting and boiling points. Because their molecules do not have an overall electric charge, these substances are not conductors. In conclusion, it's all about sharing.

SONG
ATOM: Shall we each share a pair of electrons?
OTHER ATOMS: Yes.

ATOM: Then we could all form a small molecule.
OTHER ATOMS: OK.

ATOM: We could mingle and each bring a single electron.
OTHER ATOMS: Alright.

ATOM: And all get along with a strong covalent bond.
OTHER ATOMS: Fine, whatever.

TULELA: Thanks, NNICK. So, a covalent bond is formed when two atoms share a pair of outer shell electrons so that within the molecule all atoms have full outer shells.

SUNAYANA: Let’s look at hydrogen. One electron in its one shell.

TULELA: Remember that in the first shell of any atom there are a maximum of two electrons.

SUNAYANA: And since hydrogen is more stable with two electrons in this shell, it combines with another hydrogen atom by both sharing their electron to form a hydrogen molecule – H2.

TULELA: And that sharing of electrons is the covalent bond.

SUNAYANA: Hydrogen also combines with group 7 non-metals in the same way – so hydrogen shares its one electron with one of chlorine’s seven outer shell electrons.

TULELA: Again, both atoms need one more electron to complete their outer shell. So they have one shared pair of electrons between them, which is a covalent bond.

SUNAYANA: And so they form hydrogen chloride - HCl. What about H2O, water?

TULELA: Oxygen has six outer shell electrons so needs two more to complete its shell. Hydrogen has one so needs one more. So we’d need 2 hydrogens and if each of the two hydrogens shared their electron with one oxygen, two separate covalent bonds are formed and all the atoms in the compound will have full outer shells. One more example for now? How about a molecule of oxygen gas?

SUNAYANA: Oxygen is in group 6 so an atom is two electrons short of a full outer shell. Sharing two of those outer shell electrons with another oxygen atom allows both to complete their respective shells.

TULELA: A double covalent bond is formed since two pairs of electrons are shared between the same two oxygen atoms.

SUNAYANA: If we choose some more examples, we’ll see that covalent bonds are found in molecules of non-metal elements, such as chlorine, and in compounds of non-metals such as methane – CH4.

TULELA: Even though the structures of these molecules are very simple, the covalent bonds themselves are very strong.

SUNAYANA: Just like our friendship, Tulela – simple but strong!

But although the bonds within the molecules are strong, the forces of attraction between the molecules, called intermolecular forces, are in fact very weak in comparison. And this means that covalently bonded substances tend to have low melting and boiling points because the molecules are easily parted from each other.

TULELA: So it’s not the covalent bonds which are broken but the weaker intermolecular forces.

SUNAYANA: Right – these intermolecular forces between molecules are not classed as bonds. Bonds only occur inside the molecules. Forces happen between the molecules.

TULELA: Right. And this why covalent molecules are usually gases or liquids at room temperature.

SUNAYANA: Exactly. I like to imagine it like a ‘strictly’ dance competition – cue the music.

At this competition there are lots of dancing couples who are doing their own intricate and synchronised dance routine together where the choreography requires a strong connection between the partners.

TULELA: Their own covalent bond.

SUNAYANA: Yes. However, the judges might decide that they are not impressed by one pair of dancers in particular and want them to leave. And it’s easier to remove the connected pair of the dancers rather than to break them apart individually.

TULELA: They’ve broken the weaker intermolecular force but not the strong covalent bond.

SUNAYANA: That’s right. The couple are out of the competition, but are still together to fight another day.

TULELA: Although, in bigger molecules the intermolecular forces become stronger and so the larger the covalent molecules, the higher the melting and boiling points. Nice dancing by the way, Sunayana.

What about their electric properties? We saw that ionic compounds conduct electricity in solutions or when melted because of their charged ions. So what about covalent molecules?

SUNAYANA: Well, you put your finger on it. Covalent molecules don’t have ions or free electrons and so they don’t conduct electricity.

TULELA: If you remember in the episode when we talked about ionic bonding, we showed you how you can draw dot and cross diagrams that shows you how the electrons are transferred from the outer shell of a metal to the outer shell of a non-metal? Well, we can use the same idea to show how electrons are shared between non-metals in covalent bonding molecules.

SUNAYANA: Again, we’re just interested in the outer shell electrons so we don’t need to draw all the electrons in all the shells. Let’s take hydrogen chloride as an example. Write this down or draw along. We draw a small circle representing hydrogen’s one and only shell, and a larger circle representing chlorine’s third shell, its outer shell and …. here’s the clever bit… we overlap them so that they intersect.

TULELA: A bit like a Venn diagram?

SUNAYANA: You bet. And we show the shared electrons – one dot from hydrogen and one cross from chlorine in the intersection, that part where the two circles overlap. And the other six crosses, those chlorine outer shell electrons are drawn on chlorine’s circle – two at the top, two at the bottom and two at the opposite side to the intersection.

TULELA: What about a molecule of oxygen - O2? We draw the two oxygen atoms so that the outer shells intersect as before. And because we know that oxygen has six outer shell electrons.

SUNAYANA: Because it is in group 6.

TULELA: That it shares two electrons from each atom. So, in the intersection we have two dots from the first oxygen and two crosses from the other, and the rest of the dots and crosses are drawn onto each atom’s outer shell appropriately so that each has a full outer shell of eight electrons.

SUNAYANA: Now that you’re all dots and crosses experts, after this episode have a go at drawing the dot and cross diagrams for these: a molecule of chlorine Cl2, ammonia which is NH3 and methane which is CH4. You can find some diagrams on the Bitesize website.

TULELA: Covalent bonding re-cap time, Sunayana.

SUNAYANA: Sometimes sharing is caring and that is exactly what covalent bonds do. Instead of giving away their electrons they share them, to complete the outer shells of non-metal elements and compounds.

TULELA: Covalently bonded substances may consist of small molecules with low melting and boiling points because the forces between molecules, the intermolecular forces are weak.

SUNAYANA: As the molecules get larger, their melting and boiling points increase.

TULELA: And covalent bonded substances do not conduct electricity.

SUNAYANA: You know what I’ve learned overall, Tulela?

TULELA: What’s that?

SUNAYANA: It’s good to share! Can I borrow your scarf?

TULELA: I’ll think about it.

SUNAYANA: On the next episode, we’ll look at larger covalently bonded substances with giant structures – including polymers.

TULELA: And remember there’s loads more chemistry and combined science on the Bitesize website and in other episodes in this series.

SUNAYANA: See ya!

SUNAYANA: I’m Dr Sunayana Bhargava, a scientist and poet.

TULELA: And I’m Tulela Pea, a science communicator and podcaster.

SUNAYANA: And this is Bitesize Chemistry. This is the fourth episode in an eight-part series on bonding, structure and properties.

TULELA: On this episode, we’ll look at the structures of large covalently bonded structures, such as polythene.

SUNAYANA: And diamond.

TULELA: Looking at the strength of the different bonds involved and how they relate to the properties of those substances. Remember to have a pen and paper handy to take notes and draw diagrams along the way. Let’s do it.

Quick reminder that in the previous episode, we looked at simple covalently bonded molecules, where electrons are shared between a fixed number of atoms to complete their respective outer shell. Simple molecules such as hydrogen, oxygen, water and methane are bonded that way.

However, simple molecular structures aren’t the only compounds held together by covalent bonds – giant molecular structures are as well.

SUNAYANA: In these giant structures, all the atoms in the substance are held together by strong covalent bonds. Before we get into the nitty gritty, NNICK can you give us an overview of these giant structures?

NNICK: Whilst simple covalently bonded structures are teeny tiny, other covalently bonded substances have molecules that are comparatively very large. Huge. Enormous. For example, polymers.

The forces between large polymer molecules are stronger than those between small molecules, so polymers have relatively higher melting points.

Giant covalent structures, on the other hand, can be comparatively gargantuan. Humungous. Prodigious. Jumbo. King size. Voluminous. Really big. They contain many, many, many, many atoms, each joined to adjacent atoms by covalent bonds.

SONG
It's big
It's enormous
It's a long polymer molecule
Whose atoms are bonded with covalent strength

It's immense
It's gigantic
It's a long polymer molecule
A chain that's many thousands of atoms in length

It's even bigger
It's colossal
It's a giant covalent structure
With a melting point which seems to be exceedingly high

It's gargantuan
It's humungous
It's a giant covalent structure
To which the word molecule doesn't apply

TULELA: Thanks, NNICK. So in a giant covalent structure all the atoms are bonded strongly together and this makes the structure very strong. A nice analogy is to picture a spider weaving a strong complex web with each thread representing a covalent bond. And where the threads intersect corresponds to where the atoms are in the structure. Breaking a single thread, the covalent bond,won’t affect the overall strength of the web, just as breaking one bond in a giant covalent structure doesn’t weaken the entire structure.

SUNAYANA: That’s a nice way to think about them, Tulela, unless you don’t like spiders. And unlike the links in a spider web, the individual covalent bonds are very strong. And since those covalent bonds are strong, we find that most giant covalent structures have very high melting and boiling points as a lot of energy is needed to break those bonds.

TULELA: And most giant covalent structures have no charged particles that are free to move and so this means that most cannot conduct electricity.

SUNAYANA: Although one exception to this is graphite which is a particular giant structure of carbon and we’ll look at why this is in episode 6 of the series.

TULELA: Let’s stay with carbon for the moment as it’s a very good example of how it forms different giant structures. For example, graphite as you’ve mentioned Sunayana, is where each carbon atom is covalently bonded to three others and this creates sheets of carbon atoms arranged in hexagons.

SUNAYANA: In a diamond, each carbon atom is bonded to four others in a very rigid lattice - a tetrahedral arrangement.

TULELA: And one other example that it’s useful to know is silicon dioxide which is also called silica. This is a compound found in sand with many repeating patterns of silicon and oxygen atoms linked together by covalent bonds in a regular arrangement forming a giant structure.

SUNAYANA: You can see diagrams of all of these structures on the Bitesize website.

TULELA: Hey Sunayana, I bought you a present. It’s a giant covalently bonded structure.

SUNAYANA: Ooh, a polythene bag. What’s inside? You haven’t bought me… a diamond?

TULELA: Not quite.

SUNAYANA: There’s nothing inside it, Tulela.

TULELA: I know, that’s the present. A polythene bag – a perfect example of a giant covalent structure.

SUNAYANA: As is a diamond.

TULELA: And a polythene bag is also an example of a polymer, which is a very long chain molecule made up of lots of repeating units called monomer.

SUNAYANA: And in the case of polyethene (also known as polythene) the monomer that is repeated is ethene, which has the formula C2H4. We can show this in a diagram of a long chain of repeating carbon and hydrogen atoms, which is the polymer, or we can just show the bit that repeats.

TULELA: Again, visit the Bitesize website for useful diagrams of this.

SUNAYANA: When I think about polymers and monomers, I imagine a necklace made of identical beads. Each bead represents a monomer, and the entire necklace is the polymer. Just as you can extend the necklace by adding more beads, polymers can grow by adding more monomers in a repetitive fashion.

TULELA: The intermolecular forces between polymer molecules are strong compared to the intermolecular forces between small molecules. And this is why polymers have higher melting and boiling points than simple covalent compounds, but lower melting and boiling points than giant covalent structures.

SUNAYANA: Polyethene is just one kind of polymer and there are loads of others but let’s look at why polymers are so important in chemistry.

TULELA: First up, because some polymers are man-made different types of man-made polymers are tailored to have a wide range of specific properties depending on what we want to use them for. We’ve seen that they can be very strong but many of them are also lightweight and resistant to corrosion which is useful in industries such as building cars and planes. And the production of polymers can be cost-effective, especially for large-scale manufacturing.

SUNAYANA: Sounds too good to be true. What about their disadvantages we should be aware of?

TULELA: Absolutely. There’s a concern about how biodegradable or recyclable they are.

SUNAYANA: As with all substances in chemistry, it sounds like it’s good to know more about the properties and how the substance is actually used in the real world to understand more about its potential good and bad sides.

TULELA: Quiz time! Here’s three giant structure questions for you to have a go at. We’ll give you 5 seconds on each.

SUNAYANA: Or you can pause after each question and take as long as you like.

TULELA: Question 1: What type of giant covalent structure is formed by each carbon atom bonding to four other carbon atoms in a tetrahedral arrangement?

SUNAYANA: That’ll be my diamond.

TULELA: I guess it will be. Question 2: Why does a diamond have a high melting point and hardness?

SUNAYANA: It has a giant covalent structure with strong bonds.

TULELA: And question 3: In the formation of a polymer, what is the name of the small, repeating unit that links together to create the polymer chain?

SUNAYANA: That’ll be a monomer.

TULELA: And how did you do? Don’t write in as there are no prizes, just the self-satisfaction and glory of being right.

SUNAYANA: Final summary of giant covalent structures, Tulela?

TULELA: Yep. You start.

SUNAYANA: OK. Giant covalent substances are very large structures where molecules are linked by covalent bonds.

TULELA: The strength of the bonds throughout the structures mean that they tend to have high melting and boiling points.

SUNAYANA: But most don’t conduct electricity as there are no freely moving charged particles.

TULELA: Silicon dioxide is an example as are two forms of giant carbon structures which are graphite and - go on, Sunayana…

SUNAYANA: Diamond.

TULELA: And polymers are large structure covalent compounds where chains of monomers are linked together by those strong covalent bonds.

SUNAYANA: More about covalent bonding can be found on the Bitesize website and there’s more Chemistry and combined science topics in other episodes in this series.

TULELA: In the next episode, we’ll look at the final type of bonds that we need to know about – metallic.

SUNAYANA: Thanks for listening to me, Sunayana

TULELA: And me, Tulela. Bye.

TULELA: I’m Tulela Pea, a science communicator and podcaster.

SUNAYANA: And I’m Dr Sunayana Bhargava, scientist and poet.

TULELA: And this is Bitesize Chemistry.

SUNAYANA: This is the fifth episode in an eight-part series on bonding, structure and properties. In this episode, we’re going to look at metallic bonding and properties of metals.

TULELA: And specifically how metal atoms are bonded together and how this relates to their structure and properties and how we can mix metals together to make alloys which have other really useful properties.

SUNAYANA: As always, it might be handy to write some notes or diagrams along the way, so hit pause when you need to. Don’t worry, we’ll still wait for you to hit play again.

TULELA: In previous episodes of this series, we’ve looked at the other two types of bonding we need to know about.

SUNAYANA: If you need a quick refresh, listen back to those episodes whenever you like.

TULELA: In this episode, we’re looking at metals and how these elements are bonded.

SUNAYANA: We’re all so familiar with them on a daily basis, from the jewellery that you are wearing, the electric wires charging your phone.

TULELA: Or the cutlery you use to make and eat your food.

SUNAYANA: But what, in terms of chemistry, makes a metal a metal? Grab a pen to make some notes. Time to bond with our own metallic maestro NNICK! Hi NNICK, easy question this time, what makes a metal a metal?

NNICK: Oh I love these. I don’t know, what makes a metal a metal?

SUNAYANA: No, it’s not a joke, I really want to know.

NNICK: Oh OK. What makes a metal a metal?

SONG
NNICK: Do you react to form positive ions?
ELEMENT: Yes I do.
NNICK: Then you're a metal. [ELECTRIC GUITAR]

Do you react to form positive ions?
ELEMENT: No I don't.
NNICK: Then you're not a metal. [FLUTE]

Are you found to the left and towards the bottom of the periodic table [LOW VOICE]?
ELEMENT: Yes I am.
NNICK: Then you're a metal. [ELECTRIC GUITAR]

Are you found towards the right and top of the periodic table [HIGH VOICE]?
ELEMENT: Yes I am.
NNICK: Then you're not a metal. [FLUTE]

Are you one of the majority of elements?
ELEMENT: Yes I am.
NNICK: Then you're a metal. [ELECTRIC GUITAR]

Are you a non-metal?
ELEMENT: Yes I am.
NNICK: Then you're not a metal.

TULELA: Thanks, NNICK. So metal atoms form positive ions when they lose their outer shell electrons and this gives a clue as to how they are bonded together.

SUNAYANA: Metals consist of giant structures in which the positive ions are tightly packed together in a regular pattern and where the outer shell electrons are free to move around.

TULELA: They are delocalised?

SUNAYANA: Indeed they are - and these delocalised electrons are free to move through the whole structure.

TULELA: There are strong electrostatic forces of attraction between the positive ions and the negative sea of delocalised electrons.

SUNAYANA: And this sharing of delocalised electrons is the metallic bonding that holds the atoms strongly together within the structure. I like to imagine this to be like a busy restaurant where the tables are the positive metal ions, and the waiters are the delocalised electrons moving in between the tables keeping everyone happy and the restaurant together.

TULELA: I love the way your brain works.

SUNAYANA: So, a metallic bond occurs because of those delocalised electrons moving freely around the metal ions. And they are responsible for the electrical and thermal conductivity of the metal.

TULELA: Aha, yes! If we recall that the definition of electricity is simply the flow of charge. And we can see that since those delocalised electrons do just that, they carry the electrical charge through the metal and so all metals are good electrical conductors.

SUNAYANA: They are also good conductors of heat for exactly the same reason - those delocalised electrons transfer thermal energy very easily.

TULELA: And because metallic bonds are generally strong due to the electrostatic attraction between the positive metal ions and the sea of negative electrons, most metals have high boiling and melting points.

SUNAYANA: Sounds like all metals have really amazingly strong properties. But if you just give me your copper bracelet for a moment.

TULELA: Really? OK.

SUNAYANA: I can do this really easy.

TULELA: Hey, don’t bend it.

SUNAYANA: It’s OK, I can bend it back. There you go.

TULELA: Thanks.

SUNAYANA: Pure metals are malleable, they can be easily deformed and bent because since the ions are all the same size, the layers of ions in the structure – those identical regular arrangements – can easily slide over each other. It’s a bit like bending a pack of cards where each layer slides past each other. That means that pure metals aren’t strong enough for particular uses.

TULELA: And that’s why we combine metals together to form alloys.

SUNAYANA: Right. People has been doing this for thousands of years. For example, bronze is an alloy of copper and tin, and its discovery marked the beginning of the Bronze Age which saw the replacement of pure copper tools and weapons with more durable and harder bronze ones.

TULELA: And what’s happening on an atomic level in alloys is that there are atoms of different sizes mixed together – and the structure is no longer regular like a pure metal. The different size atoms distort the layers and so greater forces are needed for those layers to slide over each other.

SUNAYANA: And so alloys are harder than pure metals – although they have all those other metallic properties – they conduct electricity and heat well because of the metallic bonding in their structure.

TULELA: And you can see some diagrams of the structures of pure metals and alloys on the Bitesize website.

SUNAYANA: Quick refresher quiz – three questions, 5 seconds each, write your answers down.

TULELA: Question 1: what type of bonding is present in metals?

SUNAYANA: It’s not a trick question – the answer is simply metallic bonding.

TULELA: Question 2: what is the key characteristic of metallic bonding that allows metals to conduct electricity?

SUNAYANA: Answer – those delocalised free moving electrons – remember those restaurant waiters.

TULELA: And question 3: copper is used in wires because one of its properties is that is it malleable. Why does it have that property?

SUNAYANA: It’s those layers of same-sized ions being able to slide over each other easily, like bending a pack of cards.

TULELA: How did you do? Don’t answer because we can’t hear you but if you need any more on metallic bonding or other topics in chemistry head over to the Bitesize website. Summary time, Sunayana?

SUNAYANA: You bet. Metals consist of giant structures of tightly packed positive ions in regular patterns and delocalised electrons.

TULELA: The negative electrons are free to move through the structure and give rise to strong metallic bonds.

SUNAYANA: Metals are good conductors of electricity and heat.

TULELA: Most metals also have high melting and boiling points.

SUNAYANA: Pure metals tend to bend easily because of their structure of regular layers of same size ions.

TULELA: Other metals are mixed to make alloys which are harder.

SUNAYANA: In the next episode, we’ll be looking at some different identities of giant structures of carbon which are called allotropes.

TULELA: Bring it on.

SUNAYANA: To hear more, search ‘Chemistry’ on BBC Sounds.

SUNAYANA: I’m Dr Sunayana Bhargava, a scientist and poet.

TULELA: And I’m Tulela Pea, a science communicator and podcaster.

SUNAYANA: And this is Bitesize Chemistry.

TULELA: This is episode six in an eight-part series on bonding, structure and properties. In this episode, we’re going to look at allotropes of carbon. First, there’s diamond – super hard and sparkly, basically atomic level bling.

SUNAYANA: There’s also graphite, its laid-back cousin.

TULELA: And then graphene? Strong and conductive which is revolutionising electronics. All allotropes of carbon.

SUNAYANA: As always, it might be handy to write some notes or diagrams along the way.

TULELA: Let’s do it.

SUNAYANA: In previous episodes, we looked at covalent bonds, how molecules and compounds share their electrons to complete their outer shell.

TULELA: And we also looked at how in giant covalent structures all the atoms are held together by covalent bonds and are arranged in giant regular lattices. So these are extremely strong structures.

SUNAYANA: Carbon is a wonderful example of an element that forms giant covalent structures. There are different forms of these structures called allotropes, depending on how the atoms are bonded covalently. First up, diamond.

TULELA: In diamond, each carbon atom contributes one electron to a shared pair with each of its four neighbouring carbon atoms. The shared electrons give rise to a tetrahedral arrangement,where each carbon atom is at the centre of the tetrahedron.

SUNAYANA: Which is a shape of a pyramid with a triangular base.

TULELA: The result is a vast network of interconnected carbon atoms, forming a rigid and robust structure where each carbon atom forms 4 covalent bonds with other carbon atoms.

SUNAYANA: So the covalent bonds in diamond are exceptionally strong, and so diamond is very hard and has a high melting point but it doesn’t conduct electricity because it doesn’t have any delocalised electrons or ions.

TULELA: i.e. no freely moving charges.

SUNAYANA: Exactly. Next up, graphite.

TULELA: In graphite, each carbon atom is covalently bonded to three of its neighbours. This creates a pattern of atoms arranged in hexagonal sheets, a bit like a honeycomb. The intermolecular forces between the sheets are fairly weak, so that they can slip over each other easily and so graphite is much softer than diamond and this is why it is often uses as a lubricant or in pencils because the layers rub off onto paper.

SUNAYANA: But even though the forces between those sheets are weak, the covalent bonds between atoms are strong, so graphite also has a high melting and boiling point.

TULELA: Right.

SUNAYANA: So in diamond, carbon shares four electrons with neighbours, in graphite it shares only three. What of that other one?

TULELA: Aha! This spare electron is delocalised between the layers which means, Sunayana?

SUNAYANA: That graphite conducts electricity.

TULELA: Exactly, it does. And this why you’ll often see graphite electrodes being used in electrical circuits. So if graphite are layers of those hexagonal patterns of carbon atoms and those layers are held together by weak forces, then graphene is simply one layer of graphite.

SUNAYANA: So like a two-dimensional very, very thin form of graphite.

TULELA: Yeah, really thin – just one atom thick. It was invented in 2004 by two scientists in the University of Manchester and you’ll never guess how they did it.

SUNAYANA: Some amazing high-tech really expensive laboratory equipment with powerful lasers?

TULELA: Nope, sticky tape.

SUNAYANA: You’re kidding.

TULELA: They used sticky tape to remove the flakes from a lump of graphite. Then used more sticky tape to make the flakes ever thinner, and then they kept going until they made flakes just one atom thick – and, bingo! Graphene.

SUNAYANA: What a beautifully simple but amazing invention. What about some of its properties?

TULELA: Well, despite its single-atom thickness, graphene is incredibly strong in fact it’s one of the strongest materials known. It’s ten times stronger than steel. Added to that, it is transparent and an excellent conductor of heat and electricity because it has delocalised electrons.

SUNAYANA: So that’s graphene. Next up are a whole family of hollow-shaped carbon structures called fullerenes. Their structures are based on hexagonal rings of carbon atoms joined by covalent bonds, although some rings can have five carbon atoms and others have seven atoms. One example of a fullerene is called a carbon nanotube.

TULELA: You can imagine a carbon nanotube as a layer of graphene rolled into a cylinder to create a tube. And because nanotubes share all the properties of graphene, they conduct electricity and heat well. And although they are very, very, light, they are also very, very strong.

They have a very high length to diameter ratio and can withstand a lot of tension without breaking. And all these properties make nanotubes useful for nanotechnology, electronics and specialised materials. You might even have a tennis racket or hockey stick which uses carbon nanotube technology.

SUNAYANA: Whilst you’re talking about sport, let’s finish with the first fullerenes to be discovered - my favourite allotrope of carbon – buckminsterfullerene.

TULELA: Say it again.

SUNAYANA: Buckminsterfullerene – just rolls off the tongue, doesn’t it? I want you to imagine a football made up of 20 hexagon patches and 12 pentagon ones all stitched together. Now shrink that down to an atomic level. At the intersection where three of those shapes meet we’re going to pop a carbon atom. We’ll find that there will be a total of sixty atoms all covalently bonded together across this ball, C60, otherwise known as ‘buckminsterfullerene’ or because of its shape, nicknamed a ‘bucky-ball’.

TULELA: So why’s it your favourite carbon allotrope?

SUNAYANA: Well, like graphene it was discovered only relatively recently, in the 1980s in fact, by a chemist at the University of Sussex - and that’s where I studied.

TULELA: And also nice that both buckminsterfullerene and graphene demonstrate that chemistry is a living breathing subject where we are continuously making new discoveries and creating new materials with amazing properties.

SUNAYANA: Quiz time! Three questions about allotropes, five seconds to write down the answers. Here goes.

TULELA: Question 1. In diamond, how many covalent bonds does each carbon atom form with its neighbouring carbon atoms?

SUNAYANA: Answer – 4.

TULELA: Question 2: Why is graphite able to conduct electricity?

SUNAYANA: Because of its delocalised electrons between the hexagonal layers.

TULELA: And question 3 – what’s the name of the carbon allotrope with sixty carbon atoms arranged in a football shape?

SUNAYANA: Buckminsterfullerene.

TULELA: Yep, exactly that.

SUNAYANA: There’s more chemistry on the Bitesize website, where you can also find diagrams of the structures of those fascinating carbon allotropes.

SUNAYANA: Carbon allotrope summary, anyone?

TULELA: Start us off.

SUNAYANA: Diamonds - in diamond, each carbon atom forms four covalent bonds with other carbon atoms, so diamond is very hard.

TULELA: Graphite. Each carbon atom forms three covalent bonds with three other carbon atoms, forming layers of hexagonal rings - one electron from each carbon atom is delocalised.

SUNAYANA: Graphene is a single layer of graphite and has properties that make it useful in electronics and composites.

TULELA: Fullerenes are molecules of carbon atoms with hollow shapes.

SUNAYANA: And the first fullerene to be discovered was Buckminsterfullerene shaped like a sphere.

TULELA: In the next episode, we’ll be looking at states of matter.

SUNAYANA: Solids, liquids and gases.

TULELA: And what it takes to go from one state to another.

SUNAYANA: To hear more, search ‘Bitesize Chemistry’ on BBC Sounds.

TULELA: Thanks for listening.

SUNAYANA: Bye.

BOTH: Buckminsterfullerene.

TULELA: I’m Tulela Pea, a science communicator and podcaster.

SUNAYANA: And I’m Dr Sunayana Bhargava, scientist and poet.

TULELA: And this is Bitesize Chemistry.

SUNAYANA: This is the seventh episode of an eight-part series on bonding, structure and properties. In this episode, we’re going to look at states of matter and changing states.

TULELA: Solids (the most dense state), liquids and gases (the least dense).

SUNAYANA: And looking at what’s going on according to the particle model. And we’ll be looking at how adding or taking energy away from a substance can convert it from one physical state to another.

TULELA: And dear podcast listening friends, grab some pen and paper and take some notes.

SUNAYANA: Look around you as you listen to this episode and everything you see, no matter where you are, is almost certainly in one of the three states of matter.

TULELA: Solid, liquid or gas.

SUNAYANA: In the Bitesize podcast studio, I can see solids which are tables, chairs, people and tea cups. But in the tea cup I can see a liquid, tea… and because the tea is still hot, I can see a gas, a trail of steam floating off into the air.

TULELA: We are so used to these three states of matter in our everyday lives, but what is going on in terms of chemistry and on a particle level – that’s what we’ll be looking after NNICK, our Bitesize binary-banter-bot gives us a quick summary of model of matter. Hi NNICK. Can you tell us about the three states of matter accordingly to the particle model?

NNICK: People say to me, "NNICK, what's the matter?" "Oh," I reply, "the matter is a collection of atoms, and it's in one of three states; solid, liquid or gas." If you heat up a solid, it melts to become a liquid. If you heat up a liquid, it boils to become a gas. Conversely, if you cool down a gas, it condenses to become a liquid. If you cool down a liquid, it freezes to become a solid. How much energy it takes to change the state of a substance depends on the strength of the forces between its particles; the stronger the forces, the higher the melting and boiling points.

SONG
Heat up a solid
It melts to become a liquid
Which can flow, but can't be compressed

Heat up a liquid
It boils to become a gas
Which can be compressed
Because each of its particles is further from the rest

Cool down a gas
It condenses to become a liquid
And now its particles have much less space

Cool down a liquid
It freezes to become a solid
Which can't flow in its space
Because each of its particles is fixed in its place

SUNAYANA: Thanks NNICK, so from what NNICK has summarised, we can visualise each individual molecule or ion or atom in the particle model as a solid sphere – like a snooker ball. And the properties of each state of matter depends on the forces between the particles.

TULELA: If we look at each in turn. In a solid, those particles are held together in a fixed rigid lattice by strong forces and each individual particle is in a low energy state.

SUNAYANA: In a snooker game, this is like the start of a game when all the red balls are tightly packed together within a triangular rack or frame with each ball touching its neighbours – no ball can swap position within the frame.

TULELA: Solids have a fixed shape and fixed volume and cannot be compressed because in this model, there is no space for the particles to move into. Let’s add some energy to the solid.

SUNAYANA: When we heat the solid, the particles gain more energy. They vibrate more and this weakens the bonds holding them together.

TULELA: In the snooker game, we’ve removed the triangular frame holding the balls rigid so that now the balls can jiggle around more and although they are free to roll around and move past each other, they do tend to stick together and remain in contact with each other.

SUNAYANA: The solid has become a liquid and the temperature at which this occurs is the melting point. Liquids don’t keep a definite shape, the particles are not in a fixed position but rather randomly arranged and they have greater energy than when they were in a solid.

TULELA: But like a solid, liquids still keep their same volume because, again, the particles are so close together that they have no space to move into. It’s like the tea in my cup, liquids flow to fill the bottom of a container. The reverse of melting, that is if we went from a liquid to a solid, is freezing.

SUNAYANA: OK. More heat, more energy. As we continue to heat our liquid, the particles move faster with more energy, weakening and breaking the bonds that hold them together, eventually overcoming the force of attraction holding them together. They are free to move, they spread out with higher energy. The liquid becomes a gas. And the temperature at which this happens is called the boiling point of the substance.

TULELA: Back on the snooker table, we can imagine this as the balls spreading out across the entire surface of the snooker table, bouncing off the cushions and each other with high speed.

SUNAYANA: Gas particles are spread out, move quickly in all directions at random and have the highest energy compared to particles in the solid and liquid states. Gases don’t keep a definite shape or volume and spread out to fill the entire space available to them, and so a gas can be compressed because the particle do have space to move into.

TULELA: A liquid becoming a gas happens at the boiling point of that liquid, but you may have heard the term evaporation which is closely connected. Both describe a liquid turning into a gas, but evaporation only happens at the surface of the liquid and can happen at any temperature.

SUNAYANA: Like the steam escaping from the top of my cup of tea.

TULELA: Whereas boiling only happens at the specific boiling point and happens throughout the entire liquid.

SUNAYANA: And the reverse of boiling – that is going back from a gas to a liquid – is called condensing.

TULELA: So quick summary, the melting point is the point at which a solid turns into a liquid if we’re heating it or a liquid becomes solid if we’re cooling it. And boiling point where a liquid changes into a gas if we’re heating it or a gas becomes a liquid if cooling.

SUNAYANA: And these points both relate directly to how strong the forces of attraction are between the particles. The stronger the force, the more energy is needed to break those bonds and so the higher the melting and boiling points are.

TULELA: If you remember from previous episodes, we saw in ionic compounds – like our friend sodium chloride - the strength of the bonds inside the ionic compound was comparatively higher than the forces between simple covalent compounds – like H2O water. And this is the reason why we found that ionic compounds have much higher melting and boiling points than simple covalent substances.

SUNAYANA: How about a states of matter example, Tulela?

TULELA: Why not indeed.

SUNAYANA: You might want to grab a pen and paper so you can write some of these numbers down. OK, so we can use the information about a substance’s melting and boiling point to predict what state it will be in at a particular temperature. If the temperature is lower than the melting point, it will be a solid. If the temperature is higher than boiling point, it will be a gas. And if the temperature is between the two, then it will be a liquid.

So I’ve got two substances here. Substance A has a melting point of minus 220 degrees Celsius and a boiling point of minus 183 degrees Celsius. And substance B has a melting point 800 degrees Celsius and a boiling point of 1400 degrees Celsius. What state of matter will each be at 1000 degrees Celsius? You can play along at home and no need to heat or cool anything with that information.

TULELA: OK – first one, substance A – melting point minus 220 degrees Celsius and boiling point minus 183 degrees Celsius. So both very low and much lower than the 1000 degrees Celsius. If we look at the boiling point, it’s lower than the temperature given so it must be a gas.

SUNAYANA: Correct.

TULELA: And substance B melting point 800 degrees Celsius, boiling point 1400 degrees Celsius and the temperature of 1000 degrees Celsius is between the two, so it must be a liquid.

SUNAYANA: Spot on.

TULELA: Summary time, Sunayana.

SUNAYANA: The three states of matter are solid, liquid and gas. Melting and freezing take place at the melting point, boiling and condensing take place at the boiling point.

TULELA: The amount of energy needed to change state from solid to liquid and from liquid to gas depends on the strength of the forces between the particles of the substance. The stronger the forces between the particles, the higher the melting point and boiling point of the substance and vice versa.

SUNAYANA: Solids have fixed shapes and cannot be compressed as the particles are not free to move.

TULELA: In liquids and gases the particles are free to move. A liquid takes the shape of its container and also cannot be compressed. And gases completely fill their container and can be compressed. And we can use information about a substances melting and boiling point to predict its state at any temperature.

SUNAYANA: As always there’s loads more guides to GCSE chemistry topics on the BBC Bitesize webpages.

TULELA: And you can listen on BBC Sounds to many other episodes in this or other podcast series of Bitesize chemistry with me, Tulela .

SUNAYANA: And me, Sunayana. Your turn, Tulela. [SOUND OF SNOOKER BALLS]

SUNAYANA: Good shot!

SUNAYANA: I’m Dr Sunayana Bhargava, a scientist and poet.

TULELA: And I’m Tulela Pea, a science communicator and podcaster.

SUNAYANA: And this is Bitesize Chemistry.

TULELA: This is the final episode in an eight-part series on bonding, structure and properties.

SUNAYANA: We’re going to be shrinking down to look at some cutting edge science on a really tiny scale. We’ll be looking at the wonderful and maybe not so wonderful world of nano particles.

TULELA: What they are, how and where we can use them as well as areas of concern over their use.

SUNAYANA: As always, it might be handy to write some notes or diagrams along the way.

For most exam boards nanoparticles is only for those doing GCSE chemistry single science so check with your teacher. However, even if this doesn’t apply to you, relax , sit back and enjoy the tiny world of nano particles with us.

TULELA: Before we get down to the nano level, let’s get some low-down on these nanoparticles with our AI chatbot… NNICK. Can you give us a summary of nano-particles?

NNICK: Nanoparticles are between 1 and 100 nanometres in size, which is of the order of a few hundred atoms. Although small, nanoparticles have a high surface area to volume ratio and so may be effective in smaller quantities than materials with larger particle sizes. They also have very big egos because of how useful they are in medicine, electronics, cosmetics and GCSE chemistry.

In conclusion:

SONG
Has anybody seen my nanoparticle?
It's very very very very small
It's smaller than the wavelength of visible light
So optically it can't be seen at all

Has anybody seen my nanoparticle?
A hundred nanometres or below
Has anybody seen my nanoparticle?
The answer is of course not, no.

SUNAYANA: Thanks NNICK! So, a couple of definitions to start with. A nanometre is one one-billionth of a metre. Or 10 to the minus 9 of a metre. And particles on the scale of one to one hundred nanometres are called nanoparticles.

TULELA: Nanoparticles are larger than atoms and molecules which are around 0.1 nanometres,

SUNAYANA: So a nanoparticle might contain only a few hundred atoms.

TULELA: If we were the size of a nanoparticle then the thickness of one sheet of paper, around 100 thousand nanometres would look like a mountain!

SUNAYANA: So I think we get the idea that nanoparticles are really really really tiny! So why do we care about them Tulela since they are so small?

TULELA: The properties of nanoparticles are different to the ones the same material would have if it was in bulk. And this is because nanoparticles are so small that they have a much higher surface area to volume ratio compared to the same material made of much larger particles. That means that the surface area of a nanoparticle is large compared to the space it takes up.

SUNAYANA: So as the size of the particle decreases, the size of the surface area increases in relation to the volume.

TULELA: A good way to think about this is that cup of tea you have there, Sunayana.

SUNAYANA: Still nice and warm. But could do with some sugar to sweeten it.

TULELA: So let’s give you one cube-shaped sugar lump.

SUNAYANA: Thanks. Takes a bit of time to dissolve and make the tea nice and sweet.

TULELA: But how about if we split the sugar lump into mini-cubes, and then split those into even smaller cubes and then split those….

SUNAYANA: Might be easier to get a spoonful from the sugar bowl Tulela…

TULELA: probably right – and in this case all the grains of sugar put together have a higher surface area to volume ratio than the large single sugar cube.

SUNAYANA: Wow that’s dissolved much faster.

TULELA: Correct – that’s because that higher surface area to volume ratio of the smaller grains means that a greater proportion of the sugar can interact faster to sweeten the tea.

SUNAYANA: Because as we decrease the lengths of each side of a sugar cube by a factor of 10, the surface area to volume ratio increases by a factor of 10. And that means the tea is in contact with more of the sugar in a shorter time.

TULELA: Correct. In the same way, the higher surface area to volume ratio of nanoparticles means that more of the atoms are available to interact with any other substances they come into contact with and so they have different properties to a larger bulk of the same substance.

SUNAYANA: And one of the amazing things about the fact that they have different properties to the same material in bulk is that this means less of it is needed when it is in nanoparticle form to get the same effect as with it in bulk.

TULELA: In one of the previous episodes we looked at the different structures of carbon…

SUNAYANA: Allotropes of carbon

TULELA: And saw that one such allotrope are a group of nanoparticles called fullerenes.

SUNAYANA: And my favourite – buckminsterfullerene!

TULELA: These have some really useful properties to make materials that are much lighter and stronger than previously possible and shows demonstrates the exciting recent advancement in the science of nanoparticles.

SUNAYANA: In nano-medicine for example nanoparticles are absorbed more easily than most larger particles to deliver drugs targeted to the right cells or wherever needed.

TULELA: In cosmetics, we have new sunscreens made from nanoparticles which have the advantage that they don’t leave that annoying streaky white mark on your skin. And other nanoparticles are used in deodorants.

SUNAYANA: Nanoparticles also make really good catalysts because of that high surface area to volume ratio. The larger the surface area, the more atoms available to interact and so the faster the rate of reaction.

TULELA: Nanoparticles has lots of wide applications in research and again shows that studying chemistry is not just about learning the periodic table but about how we can advance and improve our world today and into the future.

SUNAYANA: Absolutely. But as with all new technology we should also be aware of some of the areas of concern, especially as the nano world is a pretty recent area of study.

TULELA: Sure – we need to know more about the way that these particles affect the human body, whether for example they disperse easily or clump together once inside a cell – and this is why lots and lots of research and testing is being done in nanoscience.

SUNAYANA: As with all science, the more we make predictions from our observations, conduct experiments and learn from those results, the more we enhance our knowledge and improve the world we live in.

TULELA: That’s a nice way to end, Sunayana!

SUNAYANA: not before a quick nano-quiz Tulela.

TULELA: Go for it.

SUNAYANA: Three questions, five seconds to answer – unless you hit the pause button. Write your answers down.

TULELA: Question 1 – what is size of a nanoparticle?

SUNAYANA: Answer – around 1 to 100 nanometers.

TULELA: Question 2 – why do nanoparticles have different and more useful properties to the same material in bulk?

SUNAYANA: It’s because of the higher surface areas to volume ratio – remember how that sugar lump compares to the many grains.

TULELA: And question 3 – if we reduce the length of the sides of a cube by a factor of ten, do we increase or decrease the surface area to volume ratio? And by how much?

SUNAYANA: We increase it by a factor of ten – and remember that in nanoparticles this means there are more atoms available to interact.

TULELA: Thanks for listening to this episode and this series. Remember you can revise this and many other topics by heading over to the Bitesize website.

SUNAYANA: Or by listening on BBC Sounds to the other episodes in this and other series of this podcast.

TULELA: In the next series we’ll be looking at chemical changes, from acids and alkalis to reactions with oxygen and electrolysis.

SUNAYANA: Sounds exciting – better boil that kettle again for a really fresh cup of tea. Maybe not so sweet this time…

TULELA: Say bye Sunayana.

SUNAYANA: Bye Sunayana!

TOGETHER: Bye.