Which of the following is not a hydrogen-like species?

$Mg^{2+}$ is isoelectronic with

The electronic configuration of a dipositive ion $M^{2+}$ is $2, 8, 14$ and its atomic mass is $56$. The number of neutrons in the nucleus would be :

An isobar of ${ }_{20} Ca^{40}$ is

Consider an imaginary ion ${}_{22}^{48}X^{3-}$. The nucleus contains '$a$'$\%$ more neutrons than the number of electrons in the ion. The value of '$a$' is $.....$ [nearest integer].

Atomic weights of carbon,nitrogen and oxygen are $12, 14$ and $16$ respectively. An atom of atomic weight $14$ and nuclear charge $+6$ is an isotope of