An element's atomic number is the

a. number of protons in an atom's nucleus.

b. weight of the atom.

c. mass of the atom.

d. sum of the protons and neutrons in an atom.

The answer is a)

because two different elements can have the same weight or the sum of protons and neutrons can be the same

but what truely differs an element from another and changes its chemistry, is the proton number