Page 109 - Physical chemistry understanding our chemical world

76 INTRODUCING INTERACTIONS AND BONDS

Table 2.10 Rule for assigning oxidation numbers

1. In a binary compound, the metal has a positive oxidation number and, if a
non-metal, it has a negative oxidation number.
2. The oxidation number of a free ion equals the charge on the ion, e.g. in
−
+
Na the sodium has a +I oxidation number and chlorine in the Cl ion
− ion
has an oxidation number of −I. The oxidation number of the MnO 4
is −I, oxide O 2− is −II and the sulphate SO 4 2− ion is −II.
3. The sum of the oxidation numbers in a polyatomic ion equals the
−
oxidation number of the ions incorporated: e.g. consider MnO 4 ion.
Overall, its oxidation number is −I (because the ion’s charge is −1). Each
oxide contributes −II to this sum, so the oxidation number of the central
manganese must be +VII.
4. The oxidation number of a neutral compound is zero. The oxidation
number of an uncombined element is zero.
5. Variable oxidation numbers:
H =+I (except in the case of hydrides)
Cl =−I (except in compounds and ions containing oxygen)
O =−II (except in peroxides and superoxides)

Unfortunately, many compounds contain bonds that are a mixture of ionic and
covalent. In such a case, a formal charge as written is unlikely to represent the
actual number of charges gained or lost. For example, the complex ferrocyanide anion
[Fe(CN) ] 4− is prepared from aqueous Fe , but the central iron atom in the complex
2+
6
definitely does not bear a +2 charge (in fact, the charge is likely
to be nearer +1.5). Therefore, we employ the concept of oxidation
Numbers written as I, number. Oxidation numbers are cited with Roman numbers, so the
II, III, ... etc. are called oxidation number of the iron atom in the ferrocyanide complex
‘Roman numerals’. is +II. The IUPAC name for the complex requires the oxidation
number: we call it hexacyanoferrate (II).
Considering the changes in oxidation number during a reaction can dramatically
simplify the concept of oxidation and reduction: oxidation is an increase in oxidation
number and reduction is a decrease in oxidation number (see Chapter 7). Be aware,
though, oxidation numbers rarely correlate with the charge on an ion. For example,
consider the sulphate anion SO 2− (IV).
4
O
−
O S O

O
(IV)

The central sulphur has eight bonds. The ion has an overall charge of −2. The
oxidation number of the sulphur is therefore 8 − 2 =+6. We generally indicate oxi-
dation numbers with roman numerals, though, so we write S(VI). Table 2.10 lists the
rules required to assign an oxidation number.