Which of the following are isoelectronic species, i.e., those having the same number of electrons:
`Na^(o+), K^(o+), Mg^(2+), Ca^(2+), S^(2-), Ar`

Explanation

Isoelectronic species have the same number of electrons
Number of electrons in sodium (Na) = 11
Number of electrons in `(Na^(+) 1)`
A positive charge denotes the loss of an electron.
similarly ,
Number of electrons in `K^(+) = 18`
number of electrons in ` Mg^(2+) =10`
number of electron in `Ca^(2+) = 18 `
A negative charge denotes the gain of an electron bya species.
number of electrons in sulphur (S) = 16
number of electrons in `S^(2-) = 18 `
number of electrons in argon (Ar) = 18
Hence, the following are isoelectronic species.
(1) ` Na^(+) and Mg^(2+)` (10 electrons each)
(2)`K^(+), Ca^(2+) , S^(2-) and Ar` ( 18 electrons each )