Solid, Liquid, Gas

In a Solid, the atoms are closely packed and may move via solid state diffusion processes. In a crystalline solid, the atoms have a regular, three-dimensional spatial array - the crystal structure. In an amorphous solid, the packing density of atoms is lower and there is no long range spatial relationship between atomic positions. Solids are condensed phases with a defined volume and shape.

In a Liquid, the atomic density is similar to that of an amorphous solid. There is no long-range ordering of the atoms, and they may move in the material by diffusive and convective transport processes. Liquids are condensed phases with a defined volume but a variable shape.

In a Gas, the atoms are not bound and the gas volume is defined by the container. Atoms are free to move and mainly interact during collisions. In an ideal gas, the atomic volume is negligible compared to the volume of the gas.