Counting Total Electrons in Molecules and Polyatomic Ions

The number of molecules or ions from the following, which do not have odd number of electrons are (A) NO_2 (B) ICl_4^(-) (C) BrF_3 (D) ClO_2 (E) NO_2^(+) (F) NO

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Question type: Numerical
Exam relevance: JEE Main · Chemistry
Academic status: Answer verified against the official NTA key
Source: Previous-year question
Editorial review: 19 September 2026

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Students also ask

Can we check valence electrons instead of total electrons to determine if a species is odd-electron?

Yes, because core electrons in complete inner shells are always paired and sum to an even number. Parity of valence electrons is identical to parity of total electrons.

Answer

Accepted answer: 3

Three species ( $I C l_{4}^{-}$ , $B r F_{3}$ , and $N O_{2}^{+}$ ) have an even number of electrons, so the answer is 3.

Why each option works or fails

Step-by-step solution

Given: Given chemical species: $N O_{2}$ , $I C l_{4}^{-}$ , $B r F_{3}$ , $C l O_{2}$ , $N O_{2}^{+}$ , $N O$ .
Strategise: A species has an odd number of total electrons if the sum of atomic numbers minus the charge is odd. Equivalently, the sum of valence electrons minus the charge is odd. We need to find species that do NOT have an odd number of electrons. This means we look for species with an EVEN number of electrons.
Execute: Count total electrons for each species: 1. $N O_{2}$ : $7 + 2 (8) = 23$ (odd) 2. $I C l_{4}^{-}$ : $53 + 4 (17) + 1 = 53 + 68 + 1 = 122$ (even) 3. $B r F_{3}$ : $35 + 3 (9) = 35 + 27 = 62$ (even) 4. $C l O_{2}$ : $17 + 2 (8) = 33$ (odd) 5. $N O_{2}^{+}$ : $7 + 2 (8) - 1 = 22$ (even) 6. $N O$ : $7 + 8 = 15$ (odd) Species that do not have an odd number of electrons (i.e. even): $I C l_{4}^{-}$ , $B r F_{3}$ , $N O_{2}^{+}$ . Total count = 3.
Verify: Check valence electron counts for parity. NO2 = 5 + 12 = 17 (odd). ICl4^- = 7 + 28 + 1 = 36 (even). BrF3 = 7 + 21 = 28 (even). ClO2 = 7 + 12 = 19 (odd). NO2^+ = 5 + 12 - 1 = 16 (even). NO = 5 + 6 = 11 (odd). Exactly 3 species have even electron counts.

Counting Total Electrons in Molecules and Polyatomic Ions

What feels right?

Three species ( $I C l_{4}^{-}$ , $B r F_{3}$ , and $N O_{2}^{+}$ ) have an even number of electrons, so the answer is 3.

Why each option works or fails

Solution

01Given

02Strategise

03Execute

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Counting Total Electrons in Molecules and Polyatomic Ions

Three species (ICl4−ICl4−​, BrF3BrF3​, and NO2+NO2+​) have an even number of electrons, so the answer is 3.

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