Chemguide: Support for CIE A level Chemistry


Learning outcome 4.3(a)

This statement looks at the lattice structures of a number of solid crystals with various sorts of bonding.

Before you go on, you should find and read the statement in your copy of the syllabus.


The links below will lead you to more information than this statement asks for. You will find that as well as the actual structures, there is an explanation of why the structures lead on to some of the important physical properties of the substances. You will need this sort of information for a later statement in this section - so don't miss it out!


Statement 4.3(a)(i) is about ionic structures such as sodium chloride and magnesium oxide. You will find this explained on the page about ionic structures.

You can ignore the section about caesium chloride. You will find magnesium oxide mentioned in passing - all you need to know about it is that it has exactly the same crystal structure as sodium chloride, but that the forces holding the ions together are greater because 2+ is attracting 2-, rather than 1+ attracting 1- in the sodium chloride case.


Statement 4.3(a)(ii) is about simple molecular crystal structures such as iodine and the fullerene allotropes of carbon. You will find some of this on the page about molecular structures.

Read down as far as the structure of iodine, but leave the bit about ice for the moment. That comes in a later statement (4.3(a)(iv)).

Up to the November 2013 papers, nothing had asked about the details of the way that the iodine molecules pack together. It is probably enough to know that the crystal of iodine consists of iodine molecules packed closely together, and held in place by relatively weak van der Waals forces.

Buckminsterfullerene and other carbon-based nanoparticles

You almost certainly know about the two main allotropes of carbon - diamond and graphite. And you should also be able to define the word "allotrope".

Allotropes are two or more different forms of an element in the same physical state. In a CIE question, they wanted you to go a bit further than this by saying that this was because the different allotropes had different arrangements of the atoms.

So learn this:

Allotropes are two or more different forms of an element in the same physical state, having different atomic arrangements.

In 1985, a third allotrope of carbon was discovered, known as buckminsterfullerene or buckyballs.

Buckminsterfullerene

Buckminsterfullerene, C60, is a carbon molecule made of 60 atoms arranged like the faces of a soccer ball - in pentagons and hexagons. That is why it is commonly known as buckyballs. This is the simplest of a family of similar nanoparticles known as fullerenes.

Nanoparticles usually have sizes in the range of 1 to 100 nm, where 1 nm is 10-9 metres. For comparison purposes, the covalent radius of a carbon atom is 0.077 nm.


Note:  This diagram was taken from Wikipedia.


  • The intermolecular bonding in the solid is van der Waals dispersion forces. That means that it will have a lower melting point than diamond or graphite where you have giant covalent bonding. In fact C60 sublimes at 800 K (527°C). Diamond and graphite sublime at around 4000 K.

  • C60 is insoluble in water, but is very slightly soluble in organic solvents like benzene or methylbenzene. It doesn't dissolve in water because it can't form hydrogen bonds with the water. It can, however, form van der Waals attractions with things like benzene.

  • CIE have said in the past that it "may be more slippery than Teflon". Teflon is the brand name for PTFE - poly(tetrafluoroethene). You can think of buckyballs as being like a lot of nano-sized ball-bearings.

  • Buckyballs pack together as you would expect ball-bearings to pack - in the sort of way 12-co-ordinated metal atoms pack. See statement 4.3(a)(v) below.

Nanotubes

Imagine a single one of the sheets of carbon atoms which make up graphite. This is known as graphene. You will find a bit more about graphene in the next statement below

Now imagine that you could roll this up into a tube. Well, you can - and it gives you a carbon nanotube.


Note:  These images were adapted from this Wikipedia page.


  • These tubes have a diameter of about 1 nm, but can be millions of times longer. Individual nanotubes are the strongest and stiffest things yet discovered (according to Wikipedia) because of the very strong covalent bonds between the atoms making up the tubes.

  • However, the van der Waals attractions between individual tubes will be weaker, and so bulk material made of carbon nanotubes won't be quite so strong because of the weaker forces between the tubes.

  • Nanotubes may be open-ended (as shown in the diagram) or may be capped at one or both ends by what is essentially half a buckyball.

  • The delocalised electrons in the graphite-like structure suggests that carbon nanotubes should conduct electricity. By changing the way the nanotube is made, it is possible to get materials which are amazingly good conductors of electricity, or to get semiconductors.

  • The ability of electrons to flow through the graphite-like arrangement of carbon atoms also means that carbon nanotubes can be extremely good conductors of heat. The energy is transferred via the movement of the electrons. The conductivity of heat is excellent along the length of the tube, but relatively poor across a bundle of tubes.

  • 
    

    Statement 4.3(a)(iii) is about giant molecular crystal structures such as silicon(IV) oxide (silicon dioxide), and the diamond, graphite and graphene allotropes of carbon.

    You will find all of this apart from graphene on the page about giant covalent structures. I personally prefer the term "giant covalent structure" to "giant molecular structure", although both terms are in common use. Strictly speaking a molecule contains a definite number of atoms joined together. That isn't true of these giant structures, where the number of atoms depends on the size of the crystal.

    The structure of graphene shows that it is just a single sheet from a graphite structure, and is therefore essentially 2-dimensional. The structure extends indefinitely in both dimensions. If you tried to stack several graphene sheets on top of each other, you would just get graphite.

    
    

    Statement 4.3(a)(iv) is about hydrogen bonded structures such as ice. You will find this towards the bottom of the page about molecular structures.

    
    

    Statement 4.3(a)(v) is about metallic structures. You will find this on the page about metallic structures.

    Copper (mentioned in the syllabus) isn't specifically discussed, but it is a 12-co-ordinated metal, and so you should concentrate on the parts of the page related to 12-co-ordination. If you haven't done it recently, it might pay you to follow the link back to metallic bonding that you will find near the top of that page before you read on.

    
    

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    © Jim Clark 2010 (last modified March 2014)