How does atomic mass change in the periodic table?
Does it not increase incrementally across the Period.........?
....And increase substantively down a Group, a column of the Periodic Table?
To a good, first approximation, atomic mass is the SUM of protons, massive nuclear particles or UNIT POSITIVE ELECTRIC CHARGE, and neutrons, massive nuclear particles of ZERO ELECTRIC CHARGE. At impossibly small nuclear distances, protons and neutrons engage in the strong nuclear force, which is strong enuff at short ranges to overcome electrostatic repulsion between the like-charged protons, and bind the nucleus together.
Anyway, consult your Periodic Table; as you use it you will soon commit the masses of the common elements, H, He, C, N, O, F, Li, Na, Fe, to memory.