Scientists know that atoms can gain or lose electrons, thus acquiring a negative or positive electrical charge (determined by the number of protons minus the number of electrons). For example, if there are 4 protons and 6 electrons, the net charge is -2 (often called the valence). An ion is an atom—or group of atoms—that takes on the net electric charge, and can be positive (cation) or negative (anion). Based on the “mathematics” of losing or gaining electrons, ions can be formed or destroyed. This is one example of where math comes in handy in chemistry.