An atomic nucleus contains z protons and n neutrons. What is the mass number of this kernel?

If you add the number of neutrons to the number of protons of an atomic nucleus, you get the mass number of an atomic nucleus. The mass of the neutron is practically equal to the mass of the proton. Often they are also united by a common name – nucleon. Therefore, it can be argued that the mass number of a nucleus is equal to the number of its constituent nucleons. It is customary to denote it by the letter A. In our case, A = z + n.
Answer: z + n.