Answer:
Ions are formed when the number of valence electrons in an atom changes.
Step-by-step explanation:
Valence electrons are located in the outermost shell. In order for atoms to become stable, they must react with other atoms to gain or loose electron(s).
Atoms can only loose electrons from their outermost shell to become stable, while the other atom, reacted with, gains electron(s) for the outer shell.