Q. 20

In which of the following pairs, the ions are iso-electronic?
A. Na+, Mg2+

B. Al3+, O–

C. Na+, O2

D. N3–, Cl-

Answer :

o Electronic configurations of Na+ - 1s2 2s2 2p6 (less in one electron Na - [Ne] 3s1 ) , therefore having 10 electrons


which resembles both Mg2+ - 1s2 2s2 2p6 (less in 2 electrons , Mg – [Ne] 3s2) and


O2– - 1s2 2s2 2p6 (gain in 2 electrons , O – [He] 2s2 2p4 ) both having equal 10 electrons .


Hence , both (i) Na+, Mg2+ and iii) Na+ , O2– are example of isoelectronic electron pairs.


o Among other 2 options , (ii) Al3+ ([Ne] less in 3 electrons having total 10 electrons ), O– ( [He] 2s2 2p5 having total 9 electrons) and


(iv)N3- ( [He] 2s2 2p6 a total of 10 electrons ), Cl- ([Ne] 3s2 3p6 a total of 18 electrons) they are incorrect options.

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