An ion is an atom or group of atoms that has a positive or negative charge. Positive and negative ions form when electrons are transferred between atoms. An Ion with a positive charge is called a cation. An Ion with a negative charge is called an anion. The charge for a cation is written as a number followed by a plus sign. For example Na1+ is written as Na+. The charge for an Anion is written with a negative sign. It is like a cation but with the minus sign.