The building blocks of ordinary matter are atoms and molecules . Molecules are two or more atoms held together by a chemical bond. Atoms themselves are made up of more fundamental particles, i.e. protons, neutrons, and electrons. In a solid , the atoms and molecules are densely packed and held in place by intermolecular forces. The atoms in a solid can be modeled as being held together by tiny springs that permit them to vibrate back and forth about their equilibrium position, but not to exchange positions with other atoms.
Solids are nearly incompressible .
In a liquid the atoms and molecules are also densely packed. They cannot easily escape from one another, but they are free to move with respect to each other. Liquids are nearly incompressible. In a gas , intermolecular forces are weak and short ranged, and the atoms and molecules can move about nearly independently. Gases are compressible .