Salt (chemistry)

In chemistry, a salt or ionic compound is a chemical compound consisting of an assembly of positively charged ions (cations) and negatively charged ions (anions), which results in a compound with no net electric charge (electrically neutral). The constituent ions are held together by electrostatic forces termed ionic bonds.

The component ions in a salt can be either inorganic, such as chloride (Cl⁻), or organic, such as acetate (CH
₃COO⁻
). Each ion can be either monatomic, such as sodium (Na⁺) and chloride (Cl⁻) in sodium chloride, or polyatomic, such as ammonium (NH⁺
₄) and carbonate (CO²⁻
₃) ions in ammonium carbonate. Salts containing basic ions hydroxide (OH⁻) or oxide (O²⁻) are classified as bases, such as sodium hydroxide and potassium oxide.

Individual ions within a salt usually have multiple near neighbours, so they are not considered to be part of molecules, but instead part of a continuous three-dimensional network. Salts usually form crystalline structures when solid.

Salts composed of small ions typically have high melting and boiling points, and are hard and brittle. As solids they are almost always electrically insulating, but when melted or dissolved they become highly conductive, because the ions become mobile. Some salts have large cations, large anions, or both. In terms of their properties, such species often are more similar to organic compounds.

In 1913 the structure of sodium chloride was determined by William Henry Bragg and his son William Lawrence Bragg. This revealed that there were six equidistant nearest neighbours for each atom, demonstrating that the constituents were not arranged in molecules or finite aggregates, but instead as a network with long-range crystalline order. Many other inorganic compounds were also found to have similar structural features. These compounds were soon described as being constituted of ions rather than neutral atoms, but proof of this hypothesis was not found until the mid-1920s, when X-ray reflection experiments (which detect the density of electrons), were performed.

Principal contributors to the development of a theoretical treatment of ionic crystal structures were Max Born, Fritz Haber, Alfred Landé, Erwin Madelung, Paul Peter Ewald, and Kazimierz Fajans. Born predicted crystal energies based on the assumption of ionic constituents, which showed good correspondence to thermochemical measurements, further supporting the assumption.

Many metals such as the alkali metals react directly with the electronegative halogens gases to form salts.

Solid salts can form upon evaporation of solvent from their solutions once the solution is supersaturated and the solid compound nucleates. This process occurs widely in nature and is the means of formation of the evaporite minerals.