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Graphite

Graphite is a giant covalent substance made entirely of carbon but it has very different properties from diamond.

In graphite, each carbon atom is bonded covalently to three other carbon atoms in a planar configuration.

Each carbon atom shares only three of its four valence electrons. The remaining valence electron is delocalised. In other words, it is not associated with a particular covalent bond location.

The planes can slide over each other because there are no covalent bonds between one layer of carbon atoms and the next one. This makes graphite a soft compound.

In graphite, planes of carbon atoms are arranged in layers (left). Graphite is a black solid (right) that is commonly used as pencil lead.
In graphite, planes of carbon atoms are arranged in layers (left). Graphite is a black solid (right) that is commonly used as pencil lead.

Because of its softness, graphite is often used as a solid lubricant to reduce the friction between two surfaces.

The delocalisation of electrons allows graphite to conduct electricity.