In the two elements, ${ }_{Z_1} A^{M_1}$ and ${ }_{z_2} B^{M_2}$, the following relations are true. $M_1 \neq M_2$ and $Z_1 \neq Z_2$ but $M_1-Z_1=M_2-Z_2$. These elements are ( $M$ is atomic weight, $Z$ is atomic number)

Given, $z_1 A^{M_1}$ and $z_2 B^{M_2}$ Here, $\quad M_1 \neq M_2$ and $Z_1 \neq Z_2$ [ $M=$ atomic weight, $Z=$ atomic number $]$ But, $\quad M_1-Z_1=M_2-Z_2$ 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.