Summary:
- The northern lights aren’t caused by atoms, because atoms are not charged particles.
- Ions are atoms that have a positive or negative charge because they have unequal numbers of protons and electrons.
- The charge of an ion is indicated by a plus (+) or minus sign (-), which is written to the right of and just above the ion’s chemical symbol.
- Most ions form when atoms transfer electrons to or from other atoms or molecules.
- For example, when a sodium atom loses an electron, it becomes a positive sodium ion.
- This and some other common ions are listed with their symbols in the Table below.