* What is the atomic number of selenium? Look it up on the periodic table. The atomic number of selenium is 34. This means every selenium atom has 34 protons.

* What is the mass number? The mass number is the sum of protons and neutrons in an atom.

* Mass number = protons + neutrons

* Mass number = 34 + 31 = 65

Therefore, the mass number of a selenium atom with 31 neutrons is 65.