3.10 Expanded Valence Shells
The octet rule tells us that eight electrons fill the outer shell of an atom to give a
noble-gas ns2np6 valence-shell configuration. However, when the central atom in a
molecule has empty d-orbitals, it may be able to accommodate 10, 12, or even
more electrons. The electrons in such an expanded valence shell may be present as
lone pairs or may be used by the central atom to form additional bonds.
A Note on Good Practice: Although “expanded valence shell” is the logically
precise term, most chemists still use the term expanded octet. ■
Because the additional electrons must be accommodated in valence orbitals,
only nonmetal atoms in Period 3 or later periods can expand their valence
shells. Atoms of these elements have empty d-orbitals in the valence shell.
Another factor—possibly the main factor—in determining whether more atoms
than allowed by the octet rule can bond to a central atom is the size of that atom.
A P atom is big enough for as many as six Cl atoms to fit comfortably around it,
and PCl5 (which in the solid is PCl4
PCl6
) is a common laboratory chemical. A
N atom, though, is too small, and NCl5 is unknown. A compound that contains an
atom with more atoms attached to it than is permitted by the octet rule is called a
hypervalent compound.
Elements that can expand their valence shells commonly show variable covalence,
the ability to form different numbers of covalent bonds. Elements that have
variable covalence can form one number of bonds in some compounds and a different
number in others. Phosphorus is an example. It reacts directly with a limited
supply of chlorine to form the toxic, colorless liquid phosphorus trichloride