An isotope of mercury has 80 protons and 120 neutrons. What is the mass number of this isotope?

The mass number of an isotope is calculated by adding the number of protons and neutrons together. In the case of the mercury isotope mentioned, there are 80 protons and 120 neutrons. By performing the calculation:

80 protons + 120 neutrons = 200

Thus, the mass number of this isotope is 200. This number represents the total count of the atomic mass contributed by both protons and neutrons in the nucleus of the atom, which is essential in identifying the isotope's overall nuclear composition. Each isotope of a given element can have different numbers of neutrons, resulting in different mass numbers despite having the same number of protons.