The reason things don't sink is to do with density . A wooden raft floats on water because wood is less dense than water.
Density is mass per unit of volume.
A 'heavy' substance should really be described one which is of high density. A 'light' substance should be described as having low density. In a solid which is less dense, the particles are further apart. The density of a solid depends on the molecular weight of the molecules and atoms that make it up and on how closely packed the particles are. Particles are packed closely together when there are strong forces of attraction between them.