Question

subatomic particles found on the outermost shell & responsible for the atoms reactivity
neutrons
isotopes
ions
valence electrons

Explanation:

Valence electrons are the electrons in the outermost shell of an atom. They are involved in chemical bonding and determine the atom's reactivity. Neutrons are neutral subatomic particles in the nucleus. Isotopes are atoms of the same element with different neutron numbers. Ions are atoms or molecules with a net electric charge.

Answer:

valence electrons