Understand that the reactivity of an atom is mainly determined by how it interacts with other atoms, which depends on the electrons involved in bonding.
Recall that protons and neutrons are located in the nucleus and affect the identity and mass of the atom, but do not directly influence chemical reactivity.
Focus on the electrons, especially those in the outermost shell (valence electrons), because these electrons participate in chemical bonds and reactions.
Recognize that the number of valence electrons determines how an atom will gain, lose, or share electrons to achieve a stable electron configuration.
Conclude that the reactivity of an atom arises primarily from the number of electrons in its outermost shell, as these electrons control chemical behavior.