The nucleus of an atom contains both protons and. The number of protons in an isotope atom does not change but the number of neutrons does. 3 Ways To Calculate Atomic Mass Wikihow Teaching Chemistry Atoms And Molecules For Kids Chemistry Worksheets Your mass number is the total number of neutrons and protons within the atom. . The first thing you will need to do is find some information about. We can state the relationship between protons neutrons and mass. The mass number of an atom is calculated by adding the number of protons and neutrons. Number of protons in 1 atom of hydrogen Atomic number 1. The atomic number is equal to the number of protons. Lithium is the 3rd element of the periodic table so its atomic number is 3. You already know how to find the mass number of an isotope by adding the protons and neutrons. For neutral atoms the. So from figure 3 the number of electrons for chloride ion is 17 1