An unknown element has a mass number of 33 and an atomic number of 12. How many neutrons does the element contain?

The mass number of an atom provides the total number of neutrons and protons in the atom. Electrons, which are incredibly light, do not significantly contribute to the overall mass of an atom; recalling this fact can make it easier to remember the meaning of the mass number.
An atom’s atomic number represents the total number of protons in the atom.
The number of neutrons can then be found by calculating the difference between the mass number and the atomic number: 33−12=21