Which of the following pairs is an example of isoelectronic species?

$_{18}Ar^{40}$,$_{20}Ca^{40}$ and $_{19}K^{40}$ are

The atoms $_6C^{13}$ and $_8O^{17}$ are related to each other as .......

Among the following,the isoelectronic species is/are:
$(I)$ $O^{2-}, F^{-}, Na^{+}, Mg^{2+}$
$(II)$ $Na^{+}, Mg^{+}, Al^{3+}, F^{-}$
$(III)$ $N^{3-}, O^{2-}, F^{-}, Ne$

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?

The nuclides (a nuclide is the general name for any nuclear species) $_6C^{12}$,$_{26}Fe^{56}$,and $_{92}U^{238}$ have $12$,$56$,and $238$ nucleons respectively in their nuclei. The total number of nucleons in a nucleus is equal to: