Electrons, within an electron shell around an atom, tend to distribute themselves as far apart from each other, within the given shell, as they can (due to each one being negatively charged). The mutual repulsion is slightly reduced between two electrons, though, if they have opposite spins. Consequently, as an electron shell starts to become filled up, the electrons become organised in pairs.
Covalent bonds form between atoms by sharing electrons in their outermost electron shells, in such a way as to give the effect of filling those incomplete shells. Some of the pairs of electrons in such a shell, therefore, are already part of the original atom, and are referred to as lone pairs. The remainder are shared with the other atom, and constitute the bond itself with that atom; these form the shared pair(s).
In a large number of cases, the electrons in a shared pair are contributed, one from each atom. However, this is not always the case, leading to dative or dipolar bonds.
- Coulson, E.H., et al. (1971). Chemistry: students' book 1 (topics 1 to 12). Nuffield Foundation, Penguin Books Ltd., Harmondsworth, Middlesex, UK, pp 244-245. ISBN 14-082651-3.
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