In 1913, henry moseley discovered a way to calculate the atomic number for each element and it was determined to arrange the periodic table by atomic number and not atomic mass. why is the atomic number the better way to organize the elements of the periodic table and not organizing by atomic mass? the atomic mass can vary based on the number of neutrons the atoms of the same element have, so the atomic mass may not be in order. also, the atomic number is unique to each element. the atomic number and atomic mass are equivalent, so the atomic number is used because it is a smaller number. the atomic number can vary based on the number of neutrons the atoms of the same element have, so the atomic mass may not be in order. also, the atomic mass is unique to each element. the atomic number can be different for atoms of the same element, so it makes the patterns of the periodic table more varied.