The Foundations of Chemistry

Nonmetals in odd-numbered periodic

groups favor odd oxidation numbers,

whereas those in even-numbered

groups favor even oxidation numbers

in their compounds. The maximum

oxidation number for a representative

element is equal to its periodic group

number. For example, sulfur (Group

VIA) can form both SF 4 and SF 6.

4-6 Combination Reactions 145

number is often variable, depending on reaction conditions. For example, phosphorus
(Group VA) combines with a limited amountof chlorine to form phosphorus trichloride,
in which phosphorus exhibits the
3 oxidation state.

P 4 (s)
6Cl 2 (g)88n4PCl 3 (
) (with limited Cl 2 )(mp 112°C)

With an excess of chlorine, the product is phosphorus pentachloride, which contains phos-
phorus in the
5 oxidation state:

P 4 (s)
10Cl 2 (g)88n4PCl 5 (s) (with excess Cl 2 ) (decomposes at 167°C)

In general, a higher oxidation state of a nonmetal is formed when it reacts with an excess of
another nonmetal.There are many more reactions in which two elements combine to form
a compound (see Sections 6-7 and 6-8).

2 Compound
Element nCompound

Phosphorus in the
3 oxidation state in PCl 3 molecules can be converted to the
5 state
in PCl 5 by combination with chlorine:

PCl 3 (
)
Cl 2 (g)88nPCl 5 (s)

Likewise, sulfur in the
4 state is converted to the
6 state when SF 4 reacts with fluor-
ine to form SF 6 :

SF 4 (g)
F 2 (g)88nSF 6 (g) sulfur hexafluoride (mp 50.5°C)

Combination reactions of this type are also oxidation–reduction reactions.

3 Compound
Compound nCompound

An example of reactions in this category is the combination of calcium oxide with carbon
dioxide to produce calcium carbonate:

 3

 5

 3 
 5

 4 
 6

Phosphorus and chlorine, two nonmetals, react to

form phosphorus pentachloride, PCl 5.