A phosphite ion consists of one phosphorus atom covalently bonded to three oxygen atoms. Each oxygen atom has one extra valence electron, giving the phosphite ion an overall charge of -3. The chemical formula of a phosphite ion is PO3-.
Phosphite ions readily bond with positively charged ions to form an ionic compound. In a solution, each phosphite ion combines with three hydrogen ions to form phosphoric acid, which has the chemical formula H3PO3. Each hydrogen ion has a charge of +1, which means that the phosphoric acid has no overall charge. Compounds containing phosphite ions are sometimes used as fungicides in agriculture.