Solids

Solids are one of the fundamental states of matter characterized by a definite shape and volume. The particles are held together by interparticle forces that resist changes in shape and size, giving solids rigidity. At the microscopic level, solids can be crystalline, with a regular, repeating arrangement of atoms forming a lattice, or amorphous, lacking long-range order.

In crystalline solids, the arrangement is described by unit cells and Bravais lattices; properties often exhibit anisotropy, varying with direction. Amorphous solids, such as glasses, have short-range order but no long-range periodicity, and their mechanical properties are generally isotropic.

Common classifications by bonding include metals (metallic bonds), ionic solids, covalent network solids, and molecular solids held together by van der Waals or hydrogen bonds. These bonding types influence density, strength, hardness, melting point, and thermal and electrical conductivities. Metals tend to be dense and conductive; polymers are often flexible and low-density; ceramics are usually hard but brittle; covalent networks such as diamond have very high hardness and melting points.

Solids can deform elastically (reversible) or plastically (permanent). Their thermal behavior includes expansion on heating and the propagation of vibrational modes called phonons, which influence heat capacity and thermal conductivity. Some solids undergo phase transitions, such as melting or sublimation, while amorphous solids can experience a glass transition.

Defects such as vacancies and dislocations affect mechanical properties and diffusion. Solid-state physics and materials science study these phenomena to understand and engineer materials for technology, construction, and electronics.