What do you understand by isoelectronic species ? Name a species that will be isoelectronic with each of the following atoms or ions.
`(i) F ^(-) " " (ii)Ar " " (iii) Mg ^(2+)" " (iv) Rb^(+)`

Explanation

Ions of different elements which have the same number of electrons but different magnitude of the nuclear charge are called isoelectronic ions.
(i) `F^(-) ` has 10 (9+1) electrons . Therefore the species nitride ion `N^(3-) (7+3)` , oxide , `O^(2-) (8+2) ` neon, Ne (10+0) , sodium ion `Na^(+) (11-1)` , magnesium ion `Mg^(2+) (12-2)` aluminium ion `A1^(3+) (13-3) ` etc .
each one of which contains 10 electrons are isoelectronic with it.
(ii) Ar has 18 electrons . Therefore the species phosphide ion , `P^(3-) (15+3)` sulphide ion , `S^(2) (16 +2),` chlorine ion `(C1^(-)(17+1)` potassium ion `K^(+) (19-1)` calcium ion `Ca^(2+) (20-2)` etc. each one of which contains 18 electrons are isoelectrons with it.
(ii) `Mg^(2+)` has 10 (12-2) electrons therefore the species `N^(3-) , O^(2) , F^(-) , Ne , Na^(+) , A1^(3+)` etc, each one of which contains 10 electrons are isoelectronic with it.
(iv) `Rb^(+)` has 36 (37-1) elecrons. Therefore the species bromide ion `B^(-) (35+1)` krypton, Kr (36 + 0) and strontium `Sr^(2+) (38-2)` each one of which has 36 electrons, are isoelectronic with it.