<P> Ionic compounds containing hydrogen ions (H) are classified as acids, and those containing basic ions hydroxide (OH) or oxide (O) are classified as bases . Ionic compounds without these ions are also known as salts and can be formed by acid--base reactions . 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 . </P> <P> Ionic compounds 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 are mobilized . </P> <P> The word ion is the Greek ἰόν, ion, "going", the present participle of ἰέναι, ienai, "to go". This term was introduced by English physicist and chemist Michael Faraday in 1834 for the then - unknown species that goes from one electrode to the other through an aqueous medium . </P> <P> In 1913 the crystal structure of sodium chloride was determined by William Henry Bragg and 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 . </P>

How ions are held together by strong forces