JEE Main 2023 · 1 February · evening · Single correct · +4 / −1
Which one of the following sets of ions represents a collection of isoelectronic species?
(Given : Atomic Number : F:9,Cl:17,Na=11,Mg=12,Al=13,K=19,Ca=20,Sc=21)
- (a) N³⁻, O²⁻, F⁻, S²⁻
- (b) Ba²⁺, Sr²⁺, K⁺, Ca²⁺
- (c) Li⁺, Na⁺, Mg²⁺, Ca²⁺
- (d) K⁺, Cl⁻, Ca²⁺, Sc³⁺
Answer and solution
Answer (d) K⁺, Cl⁻, Ca²⁺, Sc³⁺
Solution
Chemistry ma’am · 3 steps
Electrons in an ion = atomic number minus charge. Find the set where every ion gives the same count.
- Count electrons. A neutral atom has Z electrons. A cation loses one electron per positive charge and an anion gains one per negative charge, so electrons = Z - (charge). Isoelectronic species all end up with the same number of electrons.
Option Electron count of each ion All equal? (a) N³⁻: 7+3=10, O²⁻: 8+2=10, F⁻: 9+1=10, S²⁻: 16+2=18 No (b) Ba²⁺: 56-2=54, Sr²⁺: 38-2=36, K⁺: 19-1=18, Ca²⁺: 20-2=18 No (c) Li⁺: 3-1=2, Na⁺: 11-1=10, Mg²⁺: 12-2=10, Ca²⁺: 20-2=18 No (d) K⁺: 19-1=18, Cl⁻: 17+1=18, Ca²⁺: 20-2=18, Sc³⁺: 21-3=18 Yes, all 18 - Shared configuration. All four ions in (d) have the argon configuration 1s² 2s² 2p⁶ 3s² 3p⁶. Each one reached 18 electrons from a different atom.
- So. Only (d) is a set of isoelectronic species, each with 18 electrons.
The idea to keep. For isoelectronic checks, do not look at charges or groups. Compute Z - charge for every ion. Isoelectronic ions usually come from atoms on both sides of a noble gas.
Answer from the NTA final answer key. Solution by PrepWiser.