In solid-state chemistry and materials science, crystalline solids are formally classified into four primary categories based on the nature of the forces and chemical bonds holding their constituent particles together: ionic solids (held by electrostatic attractions between oppositely charged ions), molecular solids (composed of neutral molecules held by weak intermolecular forces), covalent network solids (vast continuous arrays of atoms bound by strong directional covalent bonds), and metallic solids (positive metal ions immersed in a delocalized sea of valence electrons).