Each atom has three atomic particles, which are electron, neutron and proton. The number of proton an atom has determines its atomic number. For an atom to be neutral, the number of proton and electron must be equal. Mass number is defined as the summation of proton and neutron in an atom. For the question given above, the number of proton is 45, because the atomic number is 45 and the number of electron too is 45. To find the number of neutron; Mass number = proton + neutron Neutron number = Mass number - proton number Neutron number = 156 - 45 = 111. Therefore the number of neutron in element X is 111.
