Finding the mass number of an atom isn't as hard as it sounds, and it doesn't require a tiny scale. The mass number of an atom is the full number of protons and neutrons located in the nucleus of an atom. Since the mass number is an identifying trait of an element's atom, the atomic structure of each element on the periodic table has a different mass number. Finding the atomic mass number helps in identifying and classifying the atom.
Find the amount of the atom's protons. For example, an atom of helium has 2 protons.
Find the amount of the atom's neutrons. For this example, a helium atom has 2 neutrons.
Add the number of neutrons to the number of protons to find the mass numbers. For this example, 2 protons plus 2 neutrons equals 4. A helium atom has a mass number of 4.
The mass number can also be found by rounding the element's atomic weight to the nearest integer. The atomic weight is listed in the bottom of the element's box in the periodic table.