What defines the mass number of an isotope? A. the sum of the neutrons and protons B. the sum of the neutrons and electrons C. the number of neutrons D. the number of protons

The sum of neutrons and protons defines the mass number of an isotope.

Answer: Option A

Explanation:

Isotopes of an element are defined as an element that possess the same number of protons but different number of neutrons in each atom. Hence the mass number in an isotope depends upon the two sub particles which are number of nucleons (i. e) the number of protons and neutrons. For example: carbon - 12 and carbon - 13 are examples of isotopes, the carbon element with mass number as 12 and 13.