The Foundations of Chemistry

Potassium, a metal, reacts with
chlorine, a nonmetal, to form
potassium chloride, KCl. The
reaction releases energy in the form
of heat and light.

COMBINATION REACTIONS

Reactions in which two or more substances combine to form a compound are called

combination reactions.

They may involve (1) the combination of two elements to form a compound, (2) the combi-

nation of an element and a compound to form a single new compound, or (3) the

combination of two compounds to form a single new compound. Let’s examine some of

these reactions.

1 Element
Element nCompound

For this type of combination reaction, each element goes from an uncombined state, where

its oxidation state is zero, to a combined state in a compound, where its oxidation state is

not zero. Thus reactions of this type are oxidation–reduction reactions (see Section 4-5).

Metal
Nonmetal nBinary Ionic Compound

Most metals react with most nonmetals to form binary ionic compounds. The Group IA

metals combine with the Group VIIA nonmetals to form binary ioniccompounds with the

general formula MX (Section 7-2):

2M(s)
X 2 88n2(M
X)(s) MLi, Na, K, Rb, Cs

XF, Cl, Br, I

This general equation thus represents the 20 combination reactions that form the ionic

compounds listed in Table 4-11. Sodium, a silvery-white metal, combines with chlorine,

a pale green gas, to form sodium chloride, or ordinary table salt. All members of both

families undergo similar reactions.

2Na(s)
Cl 2 (g)88n2NaCl(s) sodium chloride (mp 801°C)

As we might expect, the Group IIA metals also combine with the Group VIIA nonmetals

to form binary compounds. Except for BeCl 2 , BeBr 2 , and BeI 2 , these are ionic compounds.

In general terms these combination reactions may be represented as:

M(s)
X 2 88nMX 2 (s) MBe, Mg, Ca, Sr, Ba

XF, Cl, Br, I

Consider the reaction of magnesium with fluorine to form magnesium fluoride:

Mg(s)
F 2 (g)88nMgF 2 (s) magnesium fluoride (mp 1266°C)

Because all the IIA and VIIA elements undergo similar reactions, the general equation,

written above, represents 20 reactions. We omit radium and astatine, the rare and highly

radioactive members of the families.

Nonmetal
Nonmetal nBinary Covalent Compound

When two nonmetals combine with each other they form binary covalentcompounds. In

such reactions, the oxidation number of the element with the more positive oxidation

4-6

Another important reaction of this
kind is the formation of metal oxides
(Section 6-8).

TABLE 4-11 Alkali Metal

Halides:

Compounds

Formed by

Group IA and

VIIA

Elements

LiF LiCl LiBr LiI
NaF NaCl NaBr NaI

KF KCl KBr KI
RbF RbCl RbBr RbI

CsF CsCl CsBr CsI