Final answer:
The mass number (A) of an atom is the total number of protons and neutrons in its nucleus. Isotopes are different forms of the same atom that vary only in the number of neutrons.
Step-by-step explanation:
The mass number (A) of an atom is the total number of protons and neutrons in its nucleus. The mass of the atom is a unit called the atomic mass unit (amu). One atomic mass unit is the mass of a proton, or about 1.67 × 10-27 kilograms. Because electrons have virtually no mass, just about all the mass of an atom is in its protons and neutrons. Therefore, the total number of protons and neutrons in an atom determines its mass in atomic mass units.
Isotopes are different forms of the same atom that vary only in the number of neutrons in their nucleus. Atomic nuclei with the same atomic number (proton number), but different neutron numbers, are isotopes of the same element.