Page 274 - Instant notes

H1
ELEMENTARY VALENCE THEORY

Key Notes
Valence theories attempt to describe the number, nature, strength
and geometric arrangements of chemical bonds between atoms.
Although they have been superseded by more sophisticated
theories, Lewis theory and VSEPR theory provide two
complementary approaches to bonding which remain useful for
elementary descriptions of simply bonded molecules.
Lewis theory is a primitive form of valence bond theory, with
atoms forming bonds by sharing electrons. No attempt is made to
describe the three-dimensional geometric shape of the molecule.
The main group elements tend to adopt inert gas electron
configurations (octets), although some elements, such as boron or
beryllium are energetically stable with incomplete octets. Many
larger elements display hypervalency, where it is energetically
favorable for more than eight valence electrons to be held in an
expanded octet.
Valence shell electron pair repulsion (VSEPR) theory explains
the shapes of molecules by focusing on the bonding orbitals
around each atom in isolation. VSEPR dictates that the geometry
which maximizes the distances between the electron pairs in the
orbitals is adopted. The basic geometry from the minimization of
electron-electron repulsion is modified by the differing repulsion
strengths of bonding and non-bonding pairs. In ammonia, NH 3 ,
for example, there are four valence shell pairs, giving an
underlying tetrahedral geometry, but the greater repulsive effect
of the non-bonding pair forces the bonding pairs closer to one
another than in the ideal tetrahedral geometry.
Related topics Many electron atoms Molecular orbital theory of diatomic
(G6) molecules I (H3)
Valence bond theory
(H2)

Valence theories

Valence theories attempt to describe the number, nature and strength of chemical bonds
between atoms. It also describes the geometric arrangement of the bonds, and so the