Adding the number of protons and neutrons found in the nucleus of an atom yields the mass number of that isotope.

Here's why:

* Protons: Determine the element's atomic number. All atoms of a given element have the same number of protons.

* Neutrons: Can vary within an element, creating isotopes. Isotopes of the same element have the same number of protons but different numbers of neutrons.

* Mass Number: Represents the total number of protons and neutrons in an atom's nucleus.

Example:

* Carbon-12 has 6 protons and 6 neutrons, for a mass number of 12.

* Carbon-14 has 6 protons and 8 neutrons, for a mass number of 14.

Both are isotopes of carbon, but they have different mass numbers due to the varying number of neutrons.