A measure of the degree of
oxidation of an atom in a substance. It is
defined as the charge an atom might be imagined to have when
electrons are counted according to an agreed-upon set of rules: (l)
the
oxidation state of a free element (uncombined element) is zero;
(2) for a simple (monatomic) ion, the
oxidation state is equal to the
net charge on the ion; (3) hydrogen has an
oxidation state of 1 and
oxygen has an
oxidation state of -2 when they are present in most
compounds. (Exceptions to this are that hydrogen has an
oxidation
state of -1 in hydrides of active metals, e.g.
LiH, and oxygen has an
oxidation state of -1 in
peroxides, e.g.
H2O2; (4) the algebraic
sum of
oxidation states of all atoms in a neutral molecule must be
zero, while in ions the algebraic sum of the
oxidation states of the
constituent atoms must be equal to the charge on the ion. For
example, the
oxidation states of sulfur in
H2S,
S8 (elementary
sulfur),
SO2,
SO3, and
H2SO4
are, respectively: -2, 0, +4, +6
and +6. The higher the
oxidation state of a given atom, the greater
is its degree of
oxidation; the lower the
oxidation state, the
greater is its degree of
reduction.
Source:
PAC, 1990, 62, 2167
(Glossary of atmospheric chemistry terms (Recommendations 1990))
on page 2204
NOTE:
Errors in
this entry in the printed version of the Compendium (omission of
minus signs) have been corrected.
