Group 1 elements, except lithium, form peroxides M2O2 with
excess oxygen, and potassium, rubidium and caesium will form
super oxides MO2. These per- and super- oxides are best prepared
by passing oxygen into a solution of the metal in liquid ammonia.
It is believed that the large ions Q~ and O^ are only stable in
lattices with larger cations—hence lithium (small cation) forms only
the normal oxide Li2O. The elements of Group II also form peroxides.