Compound A compound is a substance which contains two or more elements chemically combined. Element An element is a pure substance made up of atoms of the same kind Mixture A mixture is formed when two or more substances are combined physically. Ion Any atom or group of atoms that bears one or more positive or negative electrical charges. Positively charged ions are called cations and negatively charged ions, anions. Cation is a positively charged atom or group of atoms and formed when an atom or group of atoms lose electrons. Anion is a negatively charged atom or group of atoms and formed when an atom or group of atoms gain electrons.