Question

Adding the number of protons plus the number of neutrons in an atom, decides it's approximate mass, what is another word for this value?

Answer 1

atomic number

The type of atom is determined by the atomic number  i. e. the number of protons . The atomic mass of an atom is approximately the number of protons plus the number of neutrons. Typically, the atomic mass listed in a periodic table is an average, weighted by the natural abundances of different isotopes.