The Foundations of Chemistry

256 CHAPTER 6: Chemical Periodicity

and require heating to supply the energy necessary to break the strong bonds in O 2 mole-

cules. Once these reactions are initiated, most release more than enough energy to be

self-sustaining and sometimes become “red hot.”

Reactions of O 2 with Metals

In general, metallic oxides (including peroxides and superoxides) are ionic solids.The Group IA

metals combine with oxygen to form three kinds of solid ionic products called oxides,

peroxides, and superoxides. Lithium combines with oxygen to form lithium oxide.

4Li(s)O 2 (g)88n2Li 2 O(s) lithium oxide (mp1700°C)

By contrast, sodium reacts with an excess of oxygen to form sodium peroxide, Na 2 O 2 ,

rather than sodium oxide, Na 2 O, as the majorproduct.

2Na(s)O 2 (g)88nNa 2 O 2 (g) sodium peroxide (decomposes at 460°C)

Peroxidescontain the OXO^2 
, O 22 
group, in which the oxidation number of each

oxygen is 
1, whereas normal oxidessuch as lithium oxide, Li 2 O, contain oxide ions, O^2 
.

The heavier members of the family (K, Rb, Cs) react with excess oxygen to form super-

oxides.These contain the superoxide ion, O 2 
, in which the oxidation number of each

oxygen is 
^12 . The reaction with K is

K(s)O 2 (g)88nKO 2 (s) potassium superoxide (mp 430°C)

The tendency of the Group IA metals to form oxygen-rich compounds increases going

down the group. This is because cation radii increase going down the group. You can

recognize these classes of compounds as

Class Contains Ions Oxidation No. of Oxygen

normal oxides O^2 
 
 2

peroxides O 22 
 
 1

superoxides O 2 
 
^12 

The Group IIA metals react with oxygen to form normal ionic oxides, MO, but at high

pressures of oxygen the heavier ones form ionic peroxides, MO 2 (Table 6-4).

2M(s)O 2 (g)88n2(M^2 , O^2 
)(s) MBe, Mg, Ca, Sr, Ba

M(s)O 2 (g)88n(M^2 , O 22 
)(s) MCa, Sr, Ba

IA IIA

Li Na K Rb Cs Be Mg Ca Sr Ba

normal oxides Li 2 ONa 2 OK 2 ORb 2 OCs 2 O BeO MgO CaO SrO BaO

peroxides Li 2 O 2 Na 2 O 2 K 2 O 2 Rb 2 O 2 Cs 2 O 2 CaO 2 SrO 2 BaO 2

superoxides NaO 2 KO 2 RbO 2 CsO 2

TABLE 6-4 Oxygen Compounds of the IA and IIA Metals*

*The shaded compounds represent the principal products of the direct reaction of the metal with oxygen.

Beryllium reacts with oxygen only at

elevated temperatures and forms only

the normal oxide, BeO. The other

Group IIA metals form normal oxides

at moderate temperatures.