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

since the atomic number is the number of protons in the nucleus, as we move to the left on the periodic table, the electrostatic pull between the nucleus and the valence electron shell decreases. as we move down a group (vertical column), the valence shell energy level gets further and further from the nucleus.