In two elements ${}_{{\text{Z}}_{\text{1}}}{\text{A}}^{{\text{M}}_{\text{1}}} \text{and}{ }_{{\text{Z}}_{\text{2}}}{\text{B}}^{{\text{M}}_{\text{2}}}.{\text{M}}_1\ne {\text{M}}_2 \text{and} {\text{Z}}_1\ne {\text{Z}}_2$ but ${\text{M}}_1-{\text{Z}}_1={\text{M}}_2-{\text{Z}}_2.$ These elements are
 

Isotones are the atoms of different elements which have same no. of neutrons.
Ex: ${}_6{}^{14}\text{C} \text{and}{ }_8^{16}\text{O}$. The no. of neutrons = 8.