Orbital Hybridization

So far we have ignored the fact that the valence electrons that interact originally occupy different
atomic orbitals in the atoms they come from. What does it mean for an electron in the 2p subshell of
an oxygen atom to be “shared” with another atom, for example? Does it still occupy the same
dumbbell-like region of space? Does it matter whether the orbital is 2px, 2py, or 2pz? Can it not be a
valence electron from the 2s subshell instead? All these questions are addressed (or perhaps more
correctly, as you shall see, sidestepped) in the orbital hybridization picture of bonding, which
“scrambles” the atomic orbitals of the central one to form new, hybrid ones that participate in
bonding. It is not so much an alternative to VSEPR theory as an extension of it that gives a fuller
understanding of the nature of bonding.

sp HYBRIDIZATION

A molecule such as BeH 2 has a linear geometry. The two valence electrons of Be, originally in the 2s
orbital, are shared with the hydrogen atoms (which in turn share their 1s electrons with Be). In the
orbital hybridization picture, each of the two electrons in Be actually occupies an orbital that is a
mix of an s and a p orbital, called an sp hybrid orbital.

These two hybrid orbitals are oriented antiparallel to each other, and interact with the 1s orbitals of
the hydrogen atoms on each end of the molecule. This leads to the 180° bond angle predicted in the
VSEPR theory (implied by the linear geometry). The general convention adopted is to call the p
orbital that participates in hybridization the pz orbital, even though such designations are by and
large arbitrary. Also, note that we mix two atomic orbitals and end up with two hybrid orbitals: In
general, the number of atomic orbitals that “go in” has to equal the number of hybrid orbitals that
“come out.”