Page 215 - Theory and Problems of BEGINNING CHEMISTRY

204 OXIDATION AND REDUCTION [CHAP. 14

the charges on the ions. We can learn more than 200 possible oxidation numbers with relative ease by learning
the following rules. We will learn other oxidation numbers as we progress.
1. All elements when uncombined have oxidation numbers equal to 0. (Some atoms also have oxidation numbers
equal to 0 in some of their compounds, by the way.)
2. The maximum oxidation number of most atoms in its compounds is equal to its periodic group number.
There are three groups that have atoms in excess of the group number and thus are exceptions to this
rule. The coinage metals have the following maximum oxidation numbers: Cu, +2; Ag, +2 (rare); and
Au, +3. Some of the noble gases (group 0) have positive oxidation numbers. Some lanthanide and actinide
element oxidation numbers exceed +3, their nominal group number.
3. The minimum oxidation number of hydrogen is −1. That of any other nonmetallic atom is equal to its group
number minus 8. That of any metallic atom is 0.

EXAMPLE 14.5. Give three possible oxidation numbers for chlorine.
Ans. Cl is in periodic group VIIA, and so its maximum oxidation number is +7 and its minimum oxidation number is
7 − 8 =−1. It also has an oxidation number of 0 when it is a free element.

EXAMPLE 14.6. Give the possible oxidation numbers for potassium.
Ans. K can have an oxidation number of 0 when it is a free element and +1 in all its compounds. (See rule 4, Sec. 14.2.)

EXAMPLE 14.7. What is the maximum oxidation number of (a) Mn, (b) Os, (c) Ba, and (d)P?
Ans. (a) +7 (group VIIB) (b) +8 (group VIII) (c) +2 (group IIA) (d) +5 (group VA)

EXAMPLE 14.8. Can titanium (Ti) exist in an oxidation state +5?
Ans. No. Its maximum oxidation state is +4 since it is in group IVB in the periodic table.

EXAMPLE 14.9. What is the minimum oxidation state of (a)P,(b) Br, and (c)K?
Ans. (a) −3 (group number − 8 =−3) (b) −1 (group number − 8 =−1) (c) 0 (Metallic atoms do not have negative
oxidation states.)

EXAMPLE 14.10. Name one possible binary compound of (a) S and F and (b) P and O.
Ans. The more electronegative element will take the negative oxidation state. (a) The maximum oxidation state of sulfur
is +6; the only oxidation number of fluorine in its compounds is −1. Therefore, it takes six fluorine atoms to balance
one sulfur atom, and the formula is SF 6 .(b) The maximum oxidation state of phosphorus is +5; the most common
negative oxidation number of oxygen is −2. Therefore, it takes five oxygen atoms to balance two phosphorus atoms,
and the formula is P 2 O 5 .

EXAMPLE 14.11. What is the formula for the phosphorus fluoride which has phosphorus in its maximum oxidation state?
Ans. The maximum oxidation number that phosphorus can have is +5 (from group VA), and so the formula is PF 5 .

The rules above gave maximum and minimum oxidation numbers, but those might not be the only oxidation
numbers or even the most important oxidation numbers for an element. Elements of the last six groups of
the periodic table, for example, may have several oxidation numbers in their compounds, most of which vary
from one another in steps of 2. For example, the major oxidation states of chlorine in its compounds are
−1, +1, +3, +5, and +7. The transition metals have oxidation numbers that may vary from one another in
steps of 1. The inner transition elements mostly form oxidation states of +3, but the first part of the actinide
series acts more as transition elements and the elements have maximum oxidation numbers that increase from
+4 for Th to +6 for U. These generalizations are not absolute rules, but allow students to make educated
guesses about possible compound formation without exhaustive memorization. These possibilities are illustrated
in Fig. 14-1.