Giant covalent structures

Last time

Before we start: last time, in Electronic configuration, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

Same element, two worlds

A diamond is the hardest natural material, and it is clear and it does not conduct. The lead in your pencil is soft and black, and it does conduct. Both are pure carbon. The only difference is how the carbon atoms are joined together, in two different giant covalent structures.

Giant covalent structures

A giant covalent structure is a huge network of atoms, all joined by strong covalent bonds. There are no separate small molecules. The network goes on and on, through the whole crystal. Three to know: diamond, graphite, and silicon(IV) oxide.

Diamond

In diamond, every carbon atom is bonded to four other carbon atoms by strong covalent bonds. The bonds point in a tetrahedral arrangement, forming a rigid structure in three dimensions. All four outer electrons of every atom are used in bonds, so there are no free electrons.

The diamond structure

Here is the structure of diamond. Every atom has four bonds going out in different directions. Many strong covalent bonds, in all directions, make diamond extremely hard.

Graphite

In graphite, each carbon atom is bonded to only three other carbon atoms, in flat layers of hexagons. Between the layers, there are only weak forces of attraction. So the layers can slide over each other. Each carbon atom has one outer electron left over, and it is delocalised, free to move between the layers.

The graphite structure

Here is graphite. Inside each layer the bonds are strong covalent bonds. Between the layers there are only weak forces, so the layers slide, and the delocalised electrons carry the electric current.

Silicon(IV) oxide

Silicon(IV) oxide has a structure like diamond, but with oxygen atoms between the silicon atoms. Each silicon atom is bonded to four oxygen atoms, and each oxygen atom is bonded to two silicon atoms. So there are twice as many oxygen atoms as silicon atoms, and the formula is SiO₂.

The SiO₂ structure

Here is silicon(IV) oxide. Silicon with four bonds, oxygen with two. All the electrons are in bonds, so it does not conduct. It is hard, with a very high melting point.

Properties they share

They have very high melting points, because a huge number of strong covalent bonds must be broken. They are insoluble in water, and very hard, except graphite, which is soft because its layers slide. Diamond and silicon(IV) oxide do not conduct. Graphite is the only one that does.