To find the mass number of an element or isotope, add the neutrons and protons in one atom. Hydrogen is just one element with multiple isotopes occurring naturally, appearing in atomic form with just one proton (mass number 1), and two isotopes, one with one neutron and another with two.
- Look at the periodic table to find the element in question
Check out the periodic table to find the number of protons in the nucleus of the element you are studying. Find the atomic number next to the symbol for the element; this is the number of protons in the nucleus of an atom of this element. Remember that this number does not change.
- Check the number of neutrons in the atoms in question
Look at the problem you are studying; if you see a phrase such as "lithium-7," then the number 7 is the mass number of that particular isotope. Check for a number of neutrons present if that phrasing does not appear, as this gives you the second part of the mass number equation.
- Add protons and neutrons together
Combine the protons and neutrons in the isotope or element you are studying. Record the answer as the mass number.