* Atomic Number: The atomic number represents the number of protons in an atom's nucleus. Mercury (Hg) has an atomic number of 80, so it has 80 protons.
* Mass Number: The mass number is the total number of protons and neutrons in an atom's nucleus. Since the mass number is 200 and we know there are 80 protons, we can find the number of neutrons:
Number of neutrons = Mass number - Atomic number
Number of neutrons = 200 - 80
Number of neutrons = 120
Therefore, an atom of mercury with a mass number of 200 has 80 protons and 120 neutrons.
