Carbon is the first element in Group IV of the Periodic Table, with the properties listed in Table 2.1. The isolated carbon atom has an electronic configuration 1s22s22p2, composed of two electrons in the 1s orbital, the filled K shell, and the remaining four electrons distributed according to two electrons in the filled 2s orbital of the L shell and two electrons in the two half-filled 2p orbitals of the same shell. In the ground state of the carbon atom, the s orbital is spherically symmetric and the p orbital is in the shape of a dumbbell which is symmetrical about its axis. While the s orbital is non-directional, the p orbital has directional properties. The ionisation energies of the electrons in a carbon atom are very different, and they are listed in Table 2.2.

Carbon atoms bond together by sharing electron pairs that form covalent bonds. The two electrons in the very stable K shell are not involved in any bonding that takes place. The bonding can lead to various known carbon allotropes, i.e. diamond, graphite, various types of fullerene and several kinds of nanotube. Since carbon bonding occurs as a result of the overlap of atomic orbitals, one might think that a carbon atom can form only two bonds with other atoms since it has only two 2p electrons available as valence electrons.