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. 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. Many other inorganic compounds were also found to have similar structural features. 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. In 1913 the crystal structure of sodium chloride was determined by William Henry Bragg and William Lawrence Bragg. This term was introduced by physicist and chemist Michael Faraday in 1834 for the then-unknown species that goes from one electrode to the other through an aqueous medium. The word ion is the Greek ἰόν, ion, "going", the present participle of ἰέναι, ienai, "to go".
As solids they are almost always electrically insulating, but when melted or dissolved they become highly conductive, because the ions are mobilized. Ionic compounds typically have high melting and boiling points, and are hard and brittle. Ionic compounds can also be produced from their constituent ions by evaporation of their solvent, precipitation, freezing, a solid-state reaction, or the electron transfer reaction of reactive metals with reactive non-metals, such as halogen gases. Ionic compounds without these ions are also known as salts and can be formed by acid–base reactions. Ionic compounds containing basic ions hydroxide (OH −) or oxide (O 2−) are classified as bases. Ionic compounds usually form crystalline structures when solid. Individual ions within an ionic compound usually have multiple nearest neighbours, so are not considered to be part of molecules, but instead part of a continuous three-dimensional network. These can be simple ions such as the sodium (Na +) and chloride (Cl −) in sodium chloride, or polyatomic species such as the ammonium ( NH +ģ) ions in ammonium carbonate. The compound is neutral overall, but consists of positively charged ions called cations and negatively charged ions called anions. In chemistry, an ionic compound is a chemical compound composed of ions held together by electrostatic forces termed ionic bonding. The yellow stipples show the electrostatic forces.
The purple spheres represent sodium cations, Na +, and the green spheres represent chloride anions, Cl −. The crystal structure of sodium chloride, NaCl, a typical ionic compound. Chemical compound involving ionic bonding
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