have the same reactivity. decrease in atomic mass. increase in atomic number. have the same number of electrons.
I gave my students this multiple choice question: Reading from left to right, elements in the same period on the Periodic Table of the Elements

Only one answer is correct among these 4 choices:
- have the same reactivity.
- decrease in atomic mass.
- increase in atomic number.
- have the same number of electrons.

Could you tell me which one is correct?

increase in atomic number.