Isobars are atoms with

Give the relation between the following pairs of species:
$(i)$ $_{20}^{40}Ca$ and $_{19}^{40}K$
$(ii)$ $_{17}^{35}Cl$ and $_{17}^{37}Cl$
$(iii)$ $_{18}^{40}Ar$,$_{20}^{40}Ca$ and $_{19}^{40}K$
$(iv)$ $_{38}^{87}Sr$ and $_{38}^{90}Sr$

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

The ratio of the number of neutrons in $_6C^{12}$ and $_{14}Si^{28}$ is ....

Elements $A$ and $B$ have the same mass number. Their atomic numbers are $27$ and $30$ respectively. If the mass number of $A$ is $57$,find the number of neutrons in $B$.

Find the correct set with isoelectronic species.