Chemistry Reference
In-Depth Information
of electrons gained, lost, or shared is referred to as the valence of the atom. When
valence electrons are lost or partially lost by an atom, the valence number is assigned
a plus (+) sign, whereas its valence is assigned a minus (-) sign when the valence
electrons are gained or partially gained by an atom.
Oxidation numbers, sometimes called
oxidation states
, are signed numbers given
to atoms in molecules and ions to define their positive or negative character. The
oxidation number refers to the number of formal charges that an atom would have in
a molecule or polyatomic ion in the case of electrons being completely transferred in
the direction indicated by the electronegativity difference between atoms.
The oxidation state of a metal ion can be defined as the charge that the metal ion
would have in the case of a purely ionic model for the complex (Yatsimirskii 1994).
The oxidation numbers are written as +1, +2, and so on, whereas charges on ions
are written 1+, 2+, and so on. In themselves, oxidation numbers have no physi-
cal meaning; they are used to simplify tasks that are more difficult to accomplish
without them.The oxidation numbers are calculated taking into account some basic
rules:
1. All pure elements are assigned the oxidation number of zero.
2. All monatomic (single-element) ions are assigned oxidation numbers equal
to their charges.
3. Certain elements usually possess a fixed oxidation number in compounds.
• The oxidation number of O in most compounds is −2, except in perox-
ides where it is −1; in combination with fluorine, it is +2.
• The oxidation number of H in most compounds is +1, except in metal
hydrides, where it is −1.
• The oxidation number of F in all compounds is −1.
• The oxidation numbers of alkali metals (Group 1) and alkaline earth
metals (Group 2) are +1 and +2, respectively.
4. The sum of all oxidation numbers in a compound equals zero, and the sum
of oxidation numbers in a polyatomic ion equals the charge of the ion.
Table 3.1
presents the oxidation numbers of some common chemical elements from
the periodic table (Bailar 1984, Greenwood and Earnshaw 2006).
Metals have positive oxidation numbers (Table 3.1). Generally, the representative
metals have oxidation numbers equal to the group A number. The metals from the
p-block can have a second oxidation number, two units smaller than the group A
number. In the case of these elements (e.g., Tl, Sn, and Pb) the stability of the high-
est oxidation state decreases down a group due to the
ns
inert electron pair. The
penetration effect is greater for
ns
orbitals than
np
orbitals. As a consequence, the
ns
electrons are more attracted to the nucleus and more inert.
The transition metals are characterized by wide variability in oxidation numbers
that differ by one for the same metal. The highest oxidation state is less than or equal
to the group B number. The only exceptions are metals in group 1 B (11), of which
the highest oxidation number is +3. In the iron group (8, VIII B), the first element,
iron, does not achieve the maximal group oxidation state (+8), and in the cobalt
group (9, VIIIB), no metal achieves an oxidation state equal to the group number.
