The atomic number Z of an element is the number of protons it contains (and in an atom it is equal to the number of electrons). The mass number M is close to the mass of one mol of the substance but it is defined as the number of protons and neutrons in the nucleus of the atom; there, the main mass of the atom is contained. Hence, N=M-Z=74-36=38(since M counts both protons and neutrons while Z only protons). Thus, it has 38 neutrons.
