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The valence shell is the outermost shell of an atom, which contains the electrons most likely to account for the nature of any
reactions involving the atom and of the bonding interactions it has with
other atoms. Electrons in the valence shell are referred to as valence electrons. The chemist Gilbert Newton Lewis was responsible for much of the early
development of the theory of the participation of valence shell electrons in chemical bonding. Linus Pauling later generalized and extended the theory while applying insights from quantum mechanics.
In a noble gas, an atom tends to have 8 electrons in its outer shell. This
serves as the model for the octet rule which is mostly applicable to main group
elements of the second and third periods. In terms of atomic orbitals, the
electrons in the valence shell are distributed 2 in the single s orbital and 2 each in the three p
orbitals.
For organometallic compounds containing transition metals, the valence shell consists of electrons in these
s and p orbitals, as well as up to 10 additional electrons, distributed as 2 into each of 5 d
orbitals, to make a total of 18 electrons in a complete valence shell for such a compound. This is referred to as the
eighteen electron rule.
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