Final answer:
The atomic number of the atom with 51 protons is 51, and the mass number, calculated by adding the number of protons and neutrons, is 104.
Step-by-step explanation:
An atom with 51 protons, 53 neutrons, and 51 electrons has an atomic number which is equal to the number of protons, and a mass number that is the sum of the protons and neutrons in its nucleus. Therefore, the atomic number (Z) for this atom is 51, as that is the number of protons. The mass number (A) is the sum of protons and neutrons, which in this case would be 51 protons + 53 neutrons = 104. The atomic number and mass number for this atom are, respectively, 51 and 104.