The introduction of a neutron into the nuclear composition of an atom would lead to a change in

The electronic configuration of a divalent metal ion $M^{2+}$ is $2, 8, 14$ and its atomic mass is $58 \, a.m.u.$ What is the number of neutrons in its nucleus?

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)

To determine the masses of the isotopes of an element,which of the following techniques is useful?

From the following pairs of ions,which one is not an iso-electronic pair?

What is the representation of an element having a mass number of $40$ and $21$ neutrons in it?