Polonium is an element with the atomic number 84. This means it has 84 protons in its nucleus. The number of protons in an atom is also known as its atomic number. The atomic number of polonium is 84.

The mass number of an atom is the total number of protons and neutrons in its nucleus. The number of neutrons can be found by subtracting the atomic number from the mass number. The mass number of polonium is 210. Therefore, the number of neutrons is 210-84 = 126.

The mass number of polonium is 210.