Atoms tend to have 8 electrons in valence shell (octet). Lewis dot structures represent valence electrons as dots. Exceptions: incomplete octet (BF₃), expanded octet (PCl₅, SF₆).
-- NCERT Class 11 Chemistry, Ch. 4, p. 103Kossel Lewis
8 MCQs9-step worked example
Source: NCERT Chemical Bonding and Molecular StructurePYQ coverage: NEET 2023Official key: NTA-verifiedLast updated: 24 Sep 2026
Kossel Lewis, explained for NEET
The Kossel-Lewis approach is the foundational framework NEET uses to test whether you understand why atoms bond and how to represent bonding electrons. Getting this wrong cascades into errors across the entire Chemical Bonding chapter.
The core idea. Kossel (1916) observed that noble gases are exceptionally stable due to their complete outer shells — 2 electrons for helium, 8 for the rest. He proposed that atoms form ions by losing or gaining electrons to achieve this configuration. Lewis, working independently, introduced the electron-dot structure: valence electrons are shown as dots around the atomic symbol, and a shared pair of electrons between two atoms constitutes a covalent bond.
What NCERT says. NCERT Class 11 Chemistry Chapter 4, page 103, establishes the octet rule as the central principle: atoms combine to achieve 8 electrons in their valence shell (or 2 for the first period). Ionic bonding arises when electronegativity difference is large enough for complete electron transfer; covalent bonding arises when atoms share pairs.
Where aspirants lose marks. The high-frequency confusion at this topic level is conflating the octet rule with a universal law. It is a useful guideline with well-documented exceptions: incomplete octets (BF₃ — boron has only 6 electrons), expanded octets (PCl₅, SF₆ — elements in period 3+ use d-orbitals), and odd-electron species (NO, NO₂). NEET distractors exploit the assumption that every stable molecule obeys the octet rule.
A second common error is drawing Lewis structures with incorrect formal charges — placing more bonds than necessary on an atom or failing to minimise formal charge. The formal charge formula (FC = valence electrons − lone pair electrons − ½ bonding electrons) is the checkpoint.
Watch-out. When a question asks "which of the following violates the octet rule," scan for period-3+ central atoms (expanded octet) and group-13 elements (incomplete octet) before committing to an answer.
Can you answer these Kossel Lewis MCQs?
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
MCQ 1Easy RecallPractice
According to Kossel's theory, atoms form ionic bonds to achieve the electronic configuration of which group of elements?
Show answer and why every option is right or wrong
Answer: C. Kossel proposed that atoms gain or lose electrons to attain the stable electronic configuration of the nearest noble gas (NCERT Class 11 Chemistry Chapter 4, page 103).
Why A is wrong: A is wrong because halogens themselves need one more electron to achieve noble gas configuration — they are not the target configuration.
Why B is wrong: B is wrong because alkali metals have one valence electron and are themselves unstable in the Kossel framework — they lose that electron to achieve a noble gas configuration.
Why D is wrong: D is wrong because alkaline earth metals have two valence electrons and lose them to reach noble gas configuration — they are not the stable target.
MCQ 2Easy RecallPractice
In a Lewis dot structure, a lone pair refers to:
Show answer and why every option is right or wrong
Answer: A. A lone pair is a pair of valence electrons that belongs to one atom and is not shared with another atom (NCERT Class 11 Chemistry Chapter 4, page 103).
Why B is wrong: B is wrong because a shared pair between two atoms is a bond pair, not a lone pair.
Why C is wrong: C is wrong because a lone pair consists of two electrons, not one. A single unpaired electron is called a radical electron or odd electron.
Why D is wrong: D is wrong because core electrons are not shown in Lewis dot structures — only valence electrons are represented.
MCQ 3Easy RecallPractice
Which of the following is the correct Lewis dot structure representation of a double bond?
Show answer and why every option is right or wrong
Answer: D. A double bond in the Lewis framework consists of two shared electron pairs (four electrons total) between two atoms (NCERT Class 11 Chemistry Chapter 4, page 103).
Why A is wrong: A is wrong because one shared pair represents a single bond, not a double bond.
Why B is wrong: B is wrong because lone pairs by definition are not shared — they remain on one atom and do not constitute a bond.
Why C is wrong: C is wrong because three shared pairs represent a triple bond.
MCQ 4Direct ApplicationPractice
Which of the following species does NOT obey the octet rule?
Show answer and why every option is right or wrong
Answer: A. In BF₃, boron has only 6 electrons in its valence shell (3 bond pairs, 0 lone pairs), making it an incomplete-octet species. This is a standard exception to the octet rule (NCERT Class 11 Chemistry Chapter 4, page 103).
Why B is wrong: B is wrong because nitrogen in NF₃ has 3 bond pairs + 1 lone pair = 8 electrons, satisfying the octet rule.
Why C is wrong: C is wrong because carbon in CCl₄ has 4 bond pairs = 8 electrons, satisfying the octet rule.
Why D is wrong: D is wrong because oxygen in H₂O has 2 bond pairs + 2 lone pairs = 8 electrons, satisfying the octet rule.
MCQ 5Direct ApplicationPractice
The formal charge on the nitrogen atom in the ammonium ion (NH₄⁺) is:
Show answer and why every option is right or wrong
Answer: B. Formal charge = valence electrons − lone pair electrons − ½(bonding electrons). For N in NH₄⁺: FC = 5 − 0 − ½(8) = 5 − 4 = +1. Nitrogen uses all 5 valence electrons in 4 bonds (one is a coordinate bond from N donating its lone pair to H⁺), leaving zero lone pairs (NCERT Class 11 Chemistry Chapter 4).
Why A is wrong: A is wrong because applying the formal charge formula gives +1, not 0. A formal charge of 0 would require nitrogen to have at least one lone pair in this structure, which it does not.
Why C is wrong: C is wrong because a −1 formal charge would require nitrogen to have more non-bonding electrons than its valence count, which does not occur in NH₄⁺.
Why D is wrong: D is wrong because +2 would require nitrogen to have only 3 bonding electrons, but nitrogen forms 4 bonds in NH₄⁺ (8 bonding electrons).
MCQ 6Direct ApplicationPractice
SF₆ is a stable molecule despite sulfur having 12 electrons in its valence shell. This is possible because sulfur:
Show answer and why every option is right or wrong
Answer: D. Sulfur is in period 3 and has vacant 3d orbitals available, allowing it to accommodate more than 8 electrons — forming 6 bonds in SF₆ (expanded octet). Period-2 elements cannot expand their octet because no d-orbitals are available in the second shell (NCERT Class 11 Chemistry Chapter 4).
Why A is wrong: A is wrong because sulfur is in period 3, not period 2. Period-2 elements (like carbon, nitrogen, oxygen) cannot expand their octet precisely because they lack d-orbitals.
Why B is wrong: B is wrong because core electrons are too tightly bound to participate in bonding. Expanded octets use valence-shell d-orbitals, not inner-shell electrons.
Why C is wrong: C is wrong because even though SF₆ has only single bonds, sulfur still has 12 electrons around it (6 bond pairs) — exceeding the octet. The octet rule IS violated; the stability comes from d-orbital availability.
MCQ 7Concept TrapPractice
NO₂ is a paramagnetic molecule with an odd number of electrons. Within the Kossel-Lewis framework, this species is best described as:
Show answer and why every option is right or wrong
Answer: C. NO₂ has 17 valence electrons (5 from N + 6×2 from O). An odd total means at least one electron cannot be paired, making the octet rule impossible to satisfy for all atoms simultaneously. This is the "odd-electron species" exception to the octet rule (NCERT Class 11 Chemistry Chapter 4).
Why A is wrong: A is wrong because 17 is an odd number — it is impossible for all electrons to be paired. The molecule is paramagnetic precisely because of the unpaired electron.
Why B is wrong: B is wrong in its reasoning. While nitrogen does not achieve a full octet in the best Lewis structure, the primary classification is 'odd-electron species,' not simply 'incomplete octet.' Incomplete octet typically refers to electron-deficient species like BF₃ where the count is even but below 8.
Why D is wrong: D is wrong because resonance redistributes electron density among equivalent structures but cannot create an extra electron. With 17 electrons, no resonance form can satisfy the octet on all atoms.
MCQ 8Concept TrapPractice
A student draws the Lewis structure of CO₂ with single bonds between C and each O, placing 3 lone pairs on each oxygen and no lone pairs on carbon. The total electron count in this structure is 16, matching the actual valence electron count. Why is this structure still considered incorrect?
Show answer and why every option is right or wrong
Answer: B. With 2 single bonds and no lone pairs, carbon has only 4 electrons around it — an incomplete octet — and formal charge = 4 − 0 − ½(4) = +2, while each oxygen gets FC = 6 − 6 − ½(2) = −1. The charges (−1, +2, −1) sum to 0, as they must for 16 electrons on a neutral molecule, but they are not minimised and carbon lacks an octet. The correct structure (O=C=O with double bonds) gives FC = 0 on all atoms (NCERT Class 11 Chemistry Chapter 4).
Why A is wrong: A is wrong because CO₂ does have 16 valence electrons (4 from C + 6×2 from O = 16). The student's total count is correct — it is the distribution that is wrong.
Why C is wrong: C is wrong because each oxygen has 3 lone pairs (6 electrons) plus one shared pair (2 electrons) = 8, exactly an octet. The atom short of an octet is carbon.
Why D is wrong: D is wrong because C–O single bonds do exist in other molecules (e.g., methanol, CH₃OH). The issue is that using single bonds in CO₂ leads to unfavorable formal charges, not that single bonds are inherently impossible.
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How do you solve a Kossel Lewis question? A worked example
- 1
Given
• Carbon: 4 valence electrons• Nitrogen: 5 valence electrons• Charge on ion: −1 (one extra electron)• Total valence electrons: 4 + 5 + 1 = 10
- 2
Required
Lewis dot structure of CN⁻ with formal charges on C and N. Bond multiplicity.
- 3
Concept
The best Lewis structure minimises formal charges on all atoms. We try single, double, and triple bond options and compare formal charges.
- 4
Formula
Formal charge: FC = (valence electrons) − (lone pair electrons) − ½(bonding electrons)
- 5
Substitution (testing the triple bond structure)
Triple bond = 6 bonding electrons. Remaining electrons = 10 − 6 = 4, distributed as 1 lone pair on C and 1 lone pair on N.
• FC on C = 4 − 2 − ½(6) = 4 − 2 − 3 = −1• FC on N = 5 − 2 − ½(6) = 5 − 2 − 3 = 0 - 6
Calculation (compare with alternatives)
Single bond attempt: C−N with 8 remaining electrons → C gets 3 lone pairs (6e), N gets 1 lone pair (2e).
FC on C = 4 − 6 − ½(2) = −3. FC on N = 5 − 2 − ½(2) = +2. Large formal charges — rejected.
Double bond attempt: C=N with 6 remaining electrons → C gets 2 lone pairs (4e), N gets 1 lone pair (2e).
FC on C = 4 − 4 − ½(4) = −2. FC on N = 5 − 2 − ½(4) = +1. Better, but still non-zero on both atoms — not optimal.
Triple bond: FC on C = −1, FC on N = 0. Sum = −1 = charge on the ion. Formal charges are minimised.
Note: the integers 4, 5, 6, 2, 3 used in formal charge arithmetic are exact counting numbers and do not affect any significant-figure consideration. - 7
Final answer
The correct Lewis structure of CN⁻ has a triple bond between C and N, with one lone pair on each atom. Formal charges: C = −1, N = 0. The negative charge resides on carbon.
- 8
Common trap
Aspirants often stop at the double bond structure because "C=N looks familiar from organic chemistry." But in CN⁻, the double bond gives FC = −2 on C and +1 on N — violating formal charge minimisation. Always check all bond orders systematically.
- 9
Similar NEET-style question
"Calculate the formal charge on oxygen in the ozone molecule (O₃), given that the central oxygen forms one single bond and one double bond with the terminal oxygens." (Answer: central O has FC = +1; the single-bonded terminal O has FC = −1; the double-bonded terminal O has FC = 0.)
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What to remember before solving Kossel Lewis questions
More in Chemical Bonding and Molecular Structure: 4 exam traps and mistakes · 2 formulas · 3 question patterns from its other lessons.
Kossel Lewis questions from past NEET papers
1 question from NEET 2023. Answers verified against NTA official keys.
NEET 2023
12
24
31
43
NTA Answer: Option 4(final)
All 18 past-paper questions from Chemical Bonding and Molecular Structure →
Sources
NCERT refs: Class 11 Chemistry Chapter 4, p.103
Page numbers are the ones printed in the current NCERT textbook (2023 rationalised edition), unless marked pre-2023. The books are free at ncert.nic.in.
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