Question

select the correct answer.
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

Explanation:

The mass number of an isotope is a core concept in atomic structure, representing the total number of the heavy, mass-contributing particles in an atom's nucleus. Electrons have negligible mass compared to protons and neutrons, so they are not included. Protons alone define the atomic number, neutrons alone do not account for the total nuclear mass, and the sum of neutrons and electrons is irrelevant to mass number.

Answer:

A. the sum of the neutrons and protons