The smallest unit of a material made with ionic bonds is called an ionic compound. An ionic compound is a compound composed of ions, held together by electrostatic forces termed ionic bonds. The compound is neutral overall, but consists of positively charged ions called cations and negatively charged ions called anions. These can be simple ions, such as the sodium (Na+) and chloride (Cl−) ions in sodium chloride, or polyatomic ions, such as the sulfate (SO2−4) ion in calcium sulfate.