Why can graphite conduct electricity but diamonds cannot?
Both are giant structures of carbon. However, they are structurally very different.
Graphite can be described as having layers of carbon atoms stacked on top of each other. In each layer, each carbon atom is bonded to three other carbon atoms, having a shape of a trigonal planar. Each sheet is bonded to the other sheet through London forces.
Since there are only three bonds, there will be a nonbonding electron for each carbon atom. This electron is in the p-orbital and will form π bonds with the nonbonding electrons from other carbon atoms. However, this is a special π bond in that it is delocalized, i.e. shared throughout the atoms. These delocalized electrons serve as charge carriers for electricity to pass through.
On the other hand, in diamonds, each carbon atom is bonded to four other carbon atoms. All of the electrons and atoms will be fixed in place. Electricity cannot pass through easily.