Answer: The number of neutrons and protons define the mass number of an isotope.
Step-by-step explanation:
An isotope is a chemical element which is a variant of same element that differ in the number of neutrons present. As, the number of neutrons differ, the mass number of the isotopes will differ and atomic number will remain constant.
Mass number is defined as the sum of number of neutrons and protons present in an element.
Atomic Mass = Number of neutrons + Number of neutrons.
For Example: To calculate the mass number of Chlorine-35 isotope having 18 neutrons and 17 protons.
Mass number = 18 + 17 = 35 amu
Hence, the number of neutrons and protons define the mass number of an isotope.