Page 279 - Instant notes

H2
VALENCE BOND THEORY

Key Notes
Valence bond theory focuses attention on formation of the
individual bonds in molecules. Bonds are generated from
combinations of the atomic orbitals from each of the bonding
atoms, and the bond is mathematically described as a function of
these atomic orbitals. Where the resulting bond has cylindrical
symmetry about the bond axis, it is termed a σ (sigma) bond.
Elements with accessible p orbitals may generate bonds by
sidelong overlap of the orbitals to give a π (pi) bond.
The valency of many elements is greater than that predicted from
the number of unpaired atomic electrons. Promotion—raising one
electron from a pair into a higher energy orbital—creates two
additional unpaired electrons in an atom, and so increases the
valency by two. The energy required for electron promotion is
offset by the energy recouped in forming two additional chemical
bonds.
The shapes of polyatomic molecules are poorly described by pure
atomic orbitals. Deviation of actual bond angles from the angles
between pure atomic orbitals is accounted for by hybridization.
Atomic orbitals are combined into hybrid orbitals, whose shape is
3
defined by the geometry and proportion of the atomic orbitals. sp
2
hybrid orbitals are tetrahedrally arranged, sp hybrid orbitals are
arranged in a trigonal planar fashion, and sp orbitals are linearly
arranged. More complex geometric configurations may be
obtained by hybridization involving d and f orbitals.
Related topics The wave nature of matter Elementary valence theory (H1)
(G4)

The structure of the Molecular orbital theory of
hydrogen atom (G5) diatomic molecules I (H3)
Many-electron atoms (G6)

Valence bond theory of diatomic molecules

Valence bond theory is a quantum-mechanical description of molecular bonding which
focuses attention on formation of the bond itself. As for Lewis theory (see Topic H1),
bonds are generated by pairing up electrons on one atom with electrons on a second atom,
but the nature of the atomic orbitals themselves are also considered. Electrons are paired
in the course of bond formation and the spin of the individual electrons must be taken
into account. Spin pairing is a requirement that the electrons from one atom are paired