Atom is the smallest particle of an element that can take part in a chemical reaction. It is made up of the nucleus containing both protons and neutrons and energy levels containing electrons. The mass number is the sum of electrons and neutrons while the atomic number is the number of protons in a nucleus of an atom. In this case the mass number is 42 and the atomic number(protons) is 20, therefore neutrons will be 42-20 = 22.
Hence the atom of calcium has 22 neutrons in the nucleus
