(a) What do you understand by isoelectronic species ? Give the formula of a species that will be isoelectronic with the following atoms or ions :
`{:(i) F ^(-), (ii) Ar ,(iii) Mg ^(2+),(iv) Rb ^(+):}`
(b) Which of the following species are isoelectronic ?
` (i) F^(-) , (ii) Mg ^(2 + ) , (iii) Na, (iv) F, (v) Cl ^(-) `
` (vi ) Al ^(3+), (vii) Ne, (viii) S ^(2-)`
` (ix) Ca ^(2+) (x) K`

Explanation

(A) Isoelectronic specics are those which have same number of electrons.
`{:((i) F ^(-) (10 e ^(-)): O ^(2-), (ii) Ar (18 e ^(-)):Cl ^(-)), ( (iii) Mg ^(*2+)(10 e ^(-)) :Na ^(+), (iv) Rb^(+) (36e ^(-)) :Kr):}`
(b) The number of electrons in these species are :
Atom /ion No. of electrons

Thus, `O ^(2-) , Mg ^(2-) , al ^(3+)` and Ne are isoelectronic and `Cl ^(-) , S ^(2-) and Ca ^(2+) ` are isoelectronic.