What primarily determines an atom's chemical reactivity?

The chemical reactivity of an atom is primarily determined by the number of electrons in its outer shell, also known as the valence shell. Atoms strive for a stable electron configuration, often resembling the electron arrangement of the nearest noble gas, which typically involves having a full outer shell.

When an atom has fewer than eight electrons in its outer shell (with a few exceptions, such as helium which is stable with two), it tends to react with other atoms to gain, lose, or share electrons, thereby forming chemical bonds. This behavior is critical in understanding why some elements are highly reactive, such as the alkali metals, which each have one electron in their outermost shell, leading them to readily form bonds with other elements to achieve stability.

In contrast, the other aspects mentioned, such as the number of protons, the mass of the atom, and the type of chemical bonds it can form, do not directly determine reactivity. The number of protons defines the element and its identity, while the mass of the atom relates to its isotopes, which don't significantly affect the atom's reactivity. Though the types of bonds formed are ultimately a result of the outer electrons, they do not independently dictate reactivity on the same level as