Final answer:
Valence electrons are the outer-shell electrons of an atom that determine the reactivity of the atom. A full valence shell makes an atom stable, while an incomplete valence shell makes an atom reactive.
Step-by-step explanation:
Valence electrons are the outer-shell electrons of an atom. They determine the reactivity of an atom because when atoms come into contact with each other, it is their valence electrons that interact first. If an atom's valence shell is full, it is stable and unlikely to react with other atoms. But if the valence shell is not full, the atom is reactive and will tend to react with other atoms to achieve a full valence shell.