In AS , we build up on the simple facts about covalent structures we had learnt at GCSE,but with a few key additions.I will be explaining in this blog what exactly a giant covalent lattice structure(aka macromolecular structure) is and two types that you may have heard of ; diamond and graphite.
Firstly, what is a giant covalent structure? A giant covalent structure is simply molecules that are covalently bonded together in a regular structure that repeats over and over again.Giant covalent lattices form ‘crystals’ due to their structure.Diamond is an example of a giant covalent lattice.
People say diamond is a girls best friend and you usually see it on expensive engagement rings.As well as reflecting light so that the diamond can shine and sparkle all day long, what is so special about diamond? Well, let me tell you.
Number 1: There can never be liquid diamond! What’s that, no liquid diamond you say? YES, no liquid diamond.Unlike Gold and Platinum which you can heat, although lots of energy is needed (Gold needs 1,064 degrees and platinum needs 1,768 degrees to melt), there can never be such thing as liquid diamond.This is because the arrangement of diamond, which I will later explain,means that diamond not only has a high melting point but also sublimes when its melting point has been reached, meaning it turns straight from a solid to a gas , there’s no transformation into liquid in between
Diamond is a giant covalent lattice(macromolecule) that consists only of Carbon.Each Carbon atom bonds covalently to 4 other carbon atoms, forming a tetrahedral shape out of the numerous carbon atoms.The structure of diamond explains the many properties it has.It has a high melting point because there are strong covalent bonds between the carbon atoms in diamond and lots of energy is needed to overcome the covalent bond.Because of its covalent bond, it is also insoluble in solvents because its strong covalent bonds are too strong for the polar molecules of a solvent such as water to separate apart the oppositely charged ions from the lattice structure.It is also hard and is used in diamond saws .It is also a good thermal conductor because vibrations can pass easily through the macromolecule but does not conduct electricity because it does not have any delocalised(free) electrons to carry the current.
Now, let me introduce to to graphite,which is an allotrope of Carbon.If you haven’t heard what an allotrope of Carbon is;usually when you’ve got an element, it is either in a solid, liquid or gas form.With an allotrope, you can different compound made entirely of the same element and with the same state, which in this case is solid, but just with a little arrangement in its structure.That is what graphite is.
Each Carbon atom in graphite is covalently bonded to 3 other Carbon atom, meaning that there is an extra delocalised electron from each atom found in the graphites macromolecular structure.Graphite is arranged in sheets made out of hexagonal rings of carbon.The sheets are held together by weak intermolecular(van der Waals) forces that can easily be overcome , allowing the graphite layers to slide over each other, making graphite slippery and useful as a lubricant.
It is also insoluble in solvents such as water because of its strong covalent bonds that are hard to overcome by the attraction of polar molecules that make up solvents (e.g. water) and have a high melting point because of strong covalent bonds between each Carbon atom which need lots of energy to overcome., just like Diamond.However, unlike diamond, graphite can conduct electricity as well as heat (it is, in other words, a good thermal conductor).
Graphites density compared to diamond is also explained by the fact that because the graphite layers are further away than the length of the covalent bonds with Carbon in diamond, graphite is less dense than diamond.