Given, z1AM1 and z2BM2 Here, M1≠M2 and Z1≠Z2 [M= atomic weight, Z= atomic number ] But, M1−Z1=M2−Z2 We know, atomic weight = number of protons + number of neutrons and atomic number = number of protons So, number of neutrons = atomic weight - atomic weight Any of two or more species of atoms or nuclei that have same number of neutrons are known as isotone. So, both elements are isotonic.