Ions have positive or negative charge because they have lost or gained electrons. Atoms are electrically neutral because they have the same number of electrons (negatively charged particles) as protons (positively charged particles). When an atom loses one or more electrons, it becomes a positive ion. When an atom gains one or more electrons, it becomes a negative ion.

The strength of an ion's charge depends on how many electrons it has lost or gained. For example, an atom that has lost two electrons has a positive charge of 2+, while an atom that has gained two electrons has a negative charge of 2-.