Final answer:
Electrons determine chemical reactivity of atoms by participating in chemical reactions and forming ions.
Step-by-step explanation:
The particle that determines chemical reactivity is electrons.
Reactivity of atoms is dependent on how easily electrons can be removed from the atom. When atoms gain or lose electrons, they form ions, which are charged particles that participate in chemical reactions.
For example, during an oxidation-reduction reaction, also known as a redox reaction, electrons are transferred from one atom to another, resulting in the formation of new substances.