A solid is a state of matter in which atoms or molecules are tightly bound together by powerful forces thereby creating a rigid body (with a defined geometric shape and volume). In other words, a solid is a material object that is in a condensed state characterized by resistance to deformation and changes in volume. The branch of physics that deals with solids is called solid-state physics, while solid-state chemistry focuses primarily on the nature and chemical and physicochemical properties of solids. The study of solid materials falls under Materials Science and Materials Technology. The study of the mechanical behavior of the solid body is the subject of solid mechanics.
The formation of a solid occurs with the establishment of bond forces between the atoms, of such intensity as to overcome the energy of thermal agitation. These bonding forces are five, of which three are strong (ionic, covalent, and metallic bond) and two weak (hydrogen bond and Van der Waals molecular forces). All these forces are attractive until the distance between the atoms is slightly less than the diameter of the atoms themselves, and they become violently repulsive for distances between the lower atoms; for this reason, the solids are difficult to compress.