There are three main phases of matter. Solids consist of atoms that are arranged in a regular crystal lattice. The atoms are closely spaced and hence the density is high. In a liquid, atoms are more loosely arranged, in no definite crystal arrangement, and kept together by weak intermolecular forces. The density is therefore less than a solid. In a gas, individual molecules/atoms only interact weakly and the density is even less than that of a liquid.
