Which one of the following is not isoelectronic with ${O^{2-}}$?

Fluoride ion $(F^-)$ is isoelectronic with:

The nitride ion in lithium nitride is composed of:

Which number is common in isotopes?

Positive ions are formed from the neutral atom by the

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)