Atoms and ions are both particles that make up matter. An atom is the smallest unit of an element that is capable of existing on its own. It has a nucleus, which contains protons and neutrons, surrounded by electrons. Neutrons and protons have the same charge, so the atom is electrically neutral. An ion is an atom or group of atoms that has a net electric charge. This happens when atoms gain or lose electrons, giving them a positive or negative charge. Ions can be either positive or negative, depending on whether they’ve gained or lost electrons.