Giant molecular solids: diamond, graphite and silicon(IV) oxide

A giant molecular (giant covalent) solid has no separate molecules at all.

Every atom is joined to its neighbours by strong covalent bonds, and the network repeats in every direction — the whole crystal is, in effect, one giant molecule.

Breaking such a solid means breaking covalent bonds, which takes a lot of energy. That single idea explains the very high melting points.

• Each carbon atom forms 4 covalent bonds in a rigid tetrahedral arrangement.
• The network repeats in three dimensions, so the whole crystal is one giant structure.
• Consequence: extremely hard (used in cutting tools), and a very high melting point — every bond must be broken.
• It does not conduct electricity: all its electrons are locked in covalent bonds, so nothing is free to move.

• Each carbon atom forms 3 covalent bonds, joining the atoms into flat hexagonal layers.
• The fourth outer electron of each carbon is delocalised within the layer — free to move along it.
• Consequences: graphite conducts electricity (the delocalised electrons carry charge), and the layers slide over each other easily (used as a lubricant and in pencil "lead").
• The layers are only weakly attracted to one another, yet the atoms within a layer are strongly bonded, so graphite still sublimes at very high temperature rather than melting normally.

• Each silicon atom bonds to 4 oxygen atoms; each oxygen atom bridges 2 silicon atoms.
• The result is a 3D giant covalent network, similar in spirit to diamond but alternating Si and O.
• Consequence: very hard, very high melting point (about $1700\ ^\circ\text{C}$), and no electrical conduction — all electrons are in bonds.