Dmitri Mendeleev and Henry Moseley developed their periodic tables about forty years apart. During that time, many discoveries in chemistry were made. Which fact was most likely discovered between the time of Mendeleev’s table and the time of Moseley’s table that helped Moseley develop his version?

Atomic mass is the average mass of different samples of the same element.
Different elements can have similar chemical properties even if their atomic masses are different.
The number of protons in an atom is different than the atom’s total mass.
Protons strongly affect an element’s atomic mass, while the electrons in atoms do not.