Sep 24, 2020 · In scientific writing, the mass number is usually located to the upper left of an atom’s symbol. When given together with the atomic number, or the number of protons, the mass number tells an atom’s element and isotope. On the periodic table, an atom's mass number can be seen to the left of the atom symbol, in the superscript position. Aug 23, 2017 · 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.