Question

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 mendeleevs table and the time of moseleys 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 atoms total mass. protons strongly affect an elements atomic mass, while the electrons in atoms do not.

Explanation:

Mendeleev's periodic table was based on atomic mass. Moseley's table was based on atomic number (number of protons). The discovery that the number of protons (atomic number) is different from atomic mass helped Moseley develop his version. Atomic mass is average mass of isotopes (first option - known before but not key for Moseley). Second option was known (Mendeleev had some elements out of atomic - mass order due to similar properties). Fourth option - protons and neutrons affect atomic mass. The key discovery was that atomic number (number of protons) is distinct from atomic mass.

Answer:

The number of protons in an atom is different than the atom’s total mass.