When we put elements on the periodic table we use the
mass.

They are placed on the periodic table based on their atomic number.

Explanation:

In the chemistry table, you can find the molecular mass and atomic number. Atomic number represents the number of the protons of an atom, and molecular mass represents the mass of the atom. The mass of an atom is made from proton and neutron. The element is made by arranging it based on the number of protons, which was the atomic number.

When we put elements on the periodic table we use the atomic mass