<P> 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 . </P> <P> Ionic compounds can be produced from their constituent ions by evaporation, precipitation, or freezing . Reactive metals such as the alkali metals can react directly with the highly electronegative halogen gases to form an ionic product . They can also be synthesized as the product of a high temperature reaction between solids . </P> <P> If the ionic compound is soluble in a solvent, it can be obtained as a solid compound by evaporating the solvent from this electrolyte solution . As the solvent is evaporated, the ions do not go into the vapour, but stay in the remaining solution, and when they become sufficiently concentrated, nucleation occurs, and they crystallize into an ionic compound . This process occurs widely in nature, and is the means of formation of the evaporite minerals . Another method of recovering the compound from solution involves saturating a solution at high temperature and then reducing the solubility by reducing the temperature until the solution is supersaturated and the solid compound nucleates . </P> <P> Insoluble ionic compounds can be precipitated by mixing two solutions, one with the cation and one with the anion in it . Because all solutions are electrically neutral, the two solutions mixed must also contain counterions of the opposite charges . To ensure that these do not contaminate the precipitated ionic compound, it is important to ensure they do not also precipitate . If the two solutions have hydrogen ions and hydroxide ions as the counterions, they will react with one another in what is called an acid--base reaction or a neutralization reaction to form water . Alternately the counterions can be chosen to ensure that even when combined into a single solution they will remain soluble as spectator ions . </P>

How can you describe the electron charge of an ionic compound