The tendency of an element to react is closely related to
a. its atomic mass.
b. attractions between its atoms.
c. the number of valence electrons in atoms of the element.
d. the ratio of protons to neutrons in atoms of the element.

C. The number of Valence electrons,
Every atom tries to follow the Octet rule i.e To have 8 electrons in its Valence shell.
Every atom tries to accomodate 8 electrons in its Valence shell to stabilize themselves, Metals usually have 1-3 eletrons in their Valence shell which they donate to non metals so their Valnce shell has 8 electroons, (The previous will now be the Valence shell and it will be full)
Similarly Non metals have 4-7 electrons, they accept electrons from metals so they can have 8 electrons in their Valence shell.
Noble gases already have 8 electrons in their Valance shell, so they do not react and stable.

Hope it helps :)

Less stable  hope this