Carbon has four electrons in its highest energy level (outer shell). It must share four more electrons to attain a full outer shell. There are four ways that carbon atoms can do this.
- By forming four covalent bonds to four other atoms.
- By bonding covalently to two other atoms using one shared pair of electrons and a third atom using two shared pairs of electrons (a double bond)
- By bonding covalently to two other atoms by means of two shared pairs of elctrons (two double bonds – this is unusual)
- By bonding covalently to two other atoms, one using a single bond and to the other by means of a triple bond (three shared pairs)
Carbon single bonds are of the type sigma, caused by direct orbital overlap along a linear axis.