An ion is an atom that has a charge. These charges can be postive or negative and depend upon the amount of electrions lost or gained. Ions are formed when they gain or lose an election ( a change in valence electrons) An example is Li+. That is a Lithium atom with a positive charge, it has lost one electron. A cation is a positively charged ion. Li+ An anion is a negatively charged ion. F-. A Fluorine atom that has gained an electron. A polyatomic ion is a multi-atom molecule with a charge. Such as ammonia NH4+. Solution An ion is an atom that has a charge. These charges can be postive or negative and depend upon the amount of electrions lost or gained. Ions are formed when they gain or lose an election ( a change in valence electrons) An example is Li+. That is a Lithium atom with a positive charge, it has lost one electron. A cation is a positively charged ion. Li+ An anion is a negatively charged ion. F-. A Fluorine atom that has gained an electron. A polyatomic ion is a multi-atom molecule with a charge. Such as ammonia NH4+..