Hybridization in linear molecules is determined the same way as for any
other sort of molecule.
Specifically, any atom with exactly two "groups" (a group is a
lone pair of electrons or a bond) is sp hybridized. An atom with
three groups is sp2 hybridized; one with four groups is
sp3 hybridized.
Five and six groups brings in the possibility of d-orbital hybridization,
which is controversial.
For example:
- Nitrogen atoms in N2 are sp hybridized, because each
has one lone pair and one (triple) bond. The same holds for CO, carbon monoxide.
- Oxygen atoms in O2 are sp2 hybridized,
because each has two lone pairs and one (double) bond. The same is true for
nitric oxide, NO.
- Fluorine atoms in F2 are sp3 hybridized,
because each has three lone pairs and one (single) bond. The same is true
for hypochlorite, ClO-.
Notice that in each example, a bond counts as just one "group,"
whether it's single, double or triple. Each lone pair of electrons counts
as one group.
In practice, all the molecules and ions cited are considered linear because
that's the only possible geometry for two atoms! But lone pairs can have
steric effects: consider BH3 and NH3. One's planar,
one's pyramidal.
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