• 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.
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