Page 292 - Instant notes

Physical chemistry 278

Molecular orbitals in heteronuclear diatomic molecules do not
have equal mixing coefficients for corresponding atomic orbitals,
leading to unequal orbital distributions over the two nuclei.
Electrons spend more time around one atom than the other,
generating a dipole over the length of the bond. This leads to a
polar covalent bond. The element which most strongly attracts
the electrons is referred to as the more electronegative element,
and the strength of the attraction is most commonly measured
using the Pauling electronegativity scale.

A positive charge, +q and negative charge, −q, separated by a
distance R, give rise to an anelectric dipole moment, a vector
directed from the positive to the negative charge across the
molecule, with magnitude qR. The value of the dipole moment is
generally reported in debye, D, where 1 debye is equal to
3.336×10 −30 C m. The calculation of dipole moments in
polyatomic molecules may be calculated by vector addition of the
dipole moments.
Related topics The wave nature of matter Elementary valence theory (H1)
(G4)
The structure of the Valence bond theory (H2)
hydrogen atom (G5)
Molecular orbital theory of
diatomic molecules I (H3)
Many-electron atoms (G6)

Hydrogen and helium molecules

The energy levels of diatomic molecules are conventionally represented in the form of a
molecular orbital energy level diagram. The orbital is represented by a horizontal line
whose vertical position indicates the relative energy of that orbital. The atomic orbital
energy levels of the two constituent atoms are arranged either side of the molecular
orbital energy levels, usually with lines linking related molecular and atomic orbitals.
This is illustrated in Fig. 1 for the hydrogen molecule.
The molecular bonding orbital for hydrogen is generated by the linear combination
of the atomic 1s orbitals, and as there can be no distinction between the 1s orbitals from
each atom, the mixing coefficients (see Topic H3) for the orbitals are equal. This means
that the molecular orbital is composed of equal proportions of each 1s orbital, and an
electron in an orbital of this nature therefore spends equal time around each nucleus.
The bonding molecular orbital is of lower energy than that of the separate atomic
orbitals, and is therefore placed below this level on the molecular orbital diagram. The
converse is true for the antibonding orbital, which is of higher energy than both the
bonding orbital, and the isolated atomic orbitals.
As with atomic orbitals, it is possible to place a maximum of two electrons of opposite
spin into each molecular orbital, and the total number of electrons occupying the
molecular orbitals is equal to the number of electrons in the isolated species. These