What defines the mass number of an isotope? the sum of the neutrons and protons the sum of the neutrons and electrons the number of neutrons the number of protons

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

Explanation:

In any elemental isotope, the only things that will affect molar mass and mass number is the number of protons and neutrons. Electrons are not counted because we usually assume they are equal to the amount of protons and have no weight.

Protons are what gives the element its atomic number and the neutrons determine the type of isotope it is within the element.

For instance:

There can be a regular Carbon - 12

But there are isotopes like Carbon - 13 and Carbon - 14.

*The number of protons stays the same but the number of neutrons are different