Definition: Density depends on how atoms or particles are arranged in matter in a specific volume. If the particles are loosely packed together with plenty of space between them, it will have a low density and will be able to float. And if the particles are tightly packed together with no or very little space between the particles, it will have higher density. For example, wood floats on water because there is plenty of space between the atoms in the wood – that is - it has low density. An iron nail sinks in water because the atoms are tightly packed in the iron nail with very little space between them. So it will have a higher density and will sink.