Chemguide: Support for CIE A level Chemistry

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.

Learning outcomes 4.2.1 and 4.2.2

I am taking these together because the Chemguide pages you will visit don't separate them.

Statement 4.2.1(a) 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 4.2.1(b) is about simple molecular crystal structures such as iodine, buckminsterfullerene (C₆₀) and ice. You will find some of this on the page about molecular structures.

Read down as far as iodine and ice. You don't need the bits about polymers for the moment.

For iodine, 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. I haven't been able to find any trace of a question dealing with the exact way the molecules pack.

Buckminsterfullerene

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, 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.

• 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 4.2.1(d) below.

Statement 4.2.1(c) is about giant molecular crystal structures such as silicon(IV) oxide (silicon dioxide) and the diamond and graphite allotropes of carbon.

You will find all of this 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.

Statement 4.2.1(d) 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.

Learning outcome 4.2.3

This statement asks you to be able to predict the sort of structure and bonding present in a substance from information about its properties

You should find all that you will need on this Chemguide page.

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