oxides, peroxides, and superoxides

Burn the alkali metals in air and you might expect them all to make the same simple oxide, like rust on iron. They do not. Lithium makes the ordinary oxide, sodium prefers a peroxide, and potassium, rubidium and caesium go further still to a superoxide. Same family, same reaction with oxygen, three different products — and the pattern is a beautiful lesson in matching the size of a cation to the size of an anion.

Oxygen offers three negative ions of increasing size and increasing electron content: the oxide ion O2- (one oxygen, charge 2-), the peroxide ion O2 2- (two oxygens joined by a single bond, total charge 2-), and the superoxide ion O2 - (two oxygens, charge 1-, and notably an odd electron, making it paramagnetic). A small, highly charged cation like Li+ is stabilized best by the small, compact O2- ion, so lithium forms Li2O. The larger Na+ can comfortably hold the bigger peroxide ion, giving Na2O2. The very large K+, Rb+, Cs+ stabilize the even larger, lower-charged superoxide, giving KO2. In short, big cations stabilize big anions — a recurring rule of lattice energy.

These compounds matter practically: sodium peroxide and potassium superoxide both react with carbon dioxide and water to release oxygen, so KO2 is used in self-contained breathing sets, submarines, and spacecraft to scrub CO2 and regenerate breathable O2. The honest subtlety is that superoxide contains an unpaired electron and is a reactive oxygen species — the same kind of aggressive oxidant that biology must defend against with enzymes — so its chemistry is not as tame as the word oxide might suggest.