the number of valence electrons. the ratio of protons to neutrons. how tightly atoms are packed. the number of neutrons in atoms.
I gave my students this multiple choice question: The tendency of an element to react chemically is closely related to

Only one answer is correct among these 4 choices:
- the number of valence electrons.
- the ratio of protons to neutrons.
- how tightly atoms are packed.
- the number of neutrons in atoms.

Could you tell me which one is correct?

the number of valence electrons.