Which subatomic particle primarily determines an atom's reactivity?

• A. Protons
• B. Neutrons
• C. Electrons
• D. Nucleus

Answer: (C)

Reactivity hinges on the electrons, especially the valence electrons in the outermost shell. Bonding and reactions occur as atoms gain, lose, or share these electrons to reach a more stable configuration. The nucleus—made up of protons and neutrons—sets the element’s identity and mass, but it’s not what drives chemical bonding. Neutrons affect isotopes, not how atoms interact with others. So the particles that control reactivity are the electrons. For example, atoms with one valence electron tend to lose it easily, making them highly reactive, while atoms with nearly full valence shells tend to gain or share electrons. Noble gases have full shells and are notably unreactive.