${\mathrm{Hg}}^{2+}$(mercury ion) contains 80 protons, 78 electrons, and 120 neutrons. Its mass number is

The mass number of an atom or ion is the sum of its protons and neutrons. In this case, the mercury ion has 80 protons and 120 nucleons (protons + neutrons). Therefore,

Mass Number = 80 protons + 120 neutrons = 200