The qualitative expression “softness” is a chemical paraphrasing of the ease of
polarization and the larger contribution of covalency than ionicity in bonding. The
cations of alkali and alkaline earth metals as well as aluminum are hard and the cations of
mercury, copper, silver, and gold, etc. are soft. Whereas oxides are hard, sulfides and
phosphorus compounds are soft. In the minerals of the Earth’s crust, aluminum, which is
hard and oxophilic is found as an oxide, and cadmium, which is soft and chalcophilic is
found as a sulfide.