Key Fact
Tetravalency of carbon
Carbon (4 valence electrons) forms 4 covalent bonds. Catenation: ability to form chains/rings of C-C bonds. Underpins enormous diversity of organic compounds.
-- NCERT Class 11 Chemistry, Ch. 8, p. 256Tetravalency Carbon
8 MCQs9-step worked example
Source: NCERT Organic Chemistry — Some Basic PrinciplesOfficial key: NTA-verifiedLast updated: 27 Sep 2026
Tetravalency Carbon, explained for NEET
Carbon has an atomic number of 6 and electronic configuration 1s² 2s² 2p². The ground state shows two unpaired electrons in 2p, suggesting a valency of two. Yet carbon almost always forms four bonds. This is the tetravalency of carbon, and understanding why it happens — and what it enables — is the core of this topic.
Why four bonds, not two? In the excited state, one 2s electron promotes to the empty 2p orbital, giving four unpaired electrons (2s¹ 2pₓ¹ 2pᵧ¹ 2p_z¹). The energy cost of promotion is more than recovered by forming two additional bonds. This is why carbon is tetravalent, not divalent (NCERT Class 11 Chemistry, Chapter 8 (Organic Chemistry — Some Basic Principles), page 257).
What tetravalency enables:
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Catenation — Carbon forms strong C–C bonds (bond energy ~348 kJ/mol). Combined with four bonding sites per atom, this allows long chains, branched chains, and rings. Silicon also catenates, but Si–Si bonds (~226 kJ/mol) are weaker and less stable.
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Multiple bonding — Tetravalent carbon can share two or three of its four bonds with the same neighbour, forming C=C double bonds and C≡C triple bonds. This creates the vast diversity of unsaturated and aromatic compounds.
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Small atomic size — Carbon's small radius means its bonds with H, O, N, and halogens are short and strong, contributing to the stability of organic compounds.
Common confusion at NEET level: Students sometimes state carbon "has four electrons in 2p" or skip the excitation step entirely. The correct sequence is: ground state (2 unpaired) → excitation (4 unpaired) → hybridisation (sp³/sp²/sp) → four equivalent or near-equivalent bonds. Tetravalency is the starting fact that makes hybridisation meaningful — without four available half-filled orbitals, the entire framework of organic bonding collapses.
Can you answer these Tetravalency Carbon 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
The electronic configuration of carbon in the ground state is 1s² 2s² 2p². How many unpaired electrons does ground-state carbon have?
Show answer and why every option is right or wrong
Answer: B. B is correct. In 2p², two electrons occupy separate 2p orbitals with parallel spins (Hund's rule), giving two unpaired electrons. NCERT Class 11 Chemistry, Chapter 8, page 257.
Why A is wrong: A is wrong because 2p² places one electron each in two of the three 2p orbitals — both are unpaired.
Why C is wrong: C is wrong because four unpaired electrons exist only in the excited state (2s¹ 2p³), not the ground state.
Why D is wrong: D is wrong because carbon has only 6 electrons total; it cannot have 6 unpaired electrons.
MCQ 2Easy RecallPractice
Carbon is tetravalent because in the excited state its electronic configuration becomes:
Show answer and why every option is right or wrong
Answer: A. A is correct. One 2s electron is promoted to the vacant 2p orbital, giving four singly occupied orbitals (one 2s and three 2p), which accounts for carbon's tetravalency. NCERT Class 11 Chemistry, Chapter 8, page 257.
Why B is wrong: B is wrong because this is the ground-state configuration with only two unpaired electrons — it cannot explain tetravalency.
Why C is wrong: C is wrong because it places 4 electrons in 2p while keeping 2s² filled — this violates the aufbau principle and gives only 8 electrons for a 6-electron atom.
Why D is wrong: D is wrong because promoting both 2s electrons is energetically unfavourable, and total electron count here exceeds 6.
MCQ 3Easy RecallPractice
Which property of carbon is a DIRECT consequence of its tetravalency and small atomic size?
Show answer and why every option is right or wrong
Answer: B. B is correct. Four bonding sites (tetravalency) plus a small atomic radius produce short, strong covalent bonds with many elements, enabling the vast diversity of organic compounds. NCERT Class 11 Chemistry, Chapter 8, page 257.
Why A is wrong: A is wrong because carbon's electronegativity (2.55) is much lower than fluorine's (3.98); small size contributes to moderate — not extreme — electronegativity.
Why C is wrong: C is wrong because carbon's four valence electrons and moderate electronegativity favour covalent, not ionic, bonding.
Why D is wrong: D is wrong because tetravalency allows carbon to share two or three bonds with the same atom (C=C, C≡C), directly enabling multiple bonding.
MCQ 4Direct ApplicationPractice
Ground-state carbon (1s² 2s² 2p²) has two unpaired electrons, yet it forms four bonds. The additional energy required for exciting one 2s electron to 2p is compensated by:
Show answer and why every option is right or wrong
Answer: D. D is correct. Promoting a 2s electron costs energy, but forming two extra bonds releases more energy than the promotion cost, making the tetravalent state energetically favourable overall. NCERT Class 11 Chemistry, Chapter 8, page 257.
Why A is wrong: A is wrong because nuclear charge (6 protons) is fixed; electron excitation does not alter the nucleus.
Why B is wrong: B is wrong because no electron is lost; promotion is an intra-atomic rearrangement, not ionisation.
Why C is wrong: C is wrong because electronegativity is an atomic property that does not meaningfully change with a 2s→2p internal promotion.
MCQ 5Direct ApplicationPractice
Carbon forms a large number of compounds mainly due to its tetravalency and a unique property called catenation. Which of the following elements also shows catenation but to a LESSER extent than carbon?
Show answer and why every option is right or wrong
Answer: C. C is correct. Silicon is in the same group as carbon and can form Si–Si chains, but Si–Si bond energy (~226 kJ/mol) is significantly lower than C–C (~348 kJ/mol), so silicon's catenation is limited. NCERT Class 11 Chemistry, Chapter 8, page 257.
Why A is wrong: A is wrong because sodium is an alkali metal that forms ionic bonds and does not exhibit catenation.
Why B is wrong: B is wrong because neon is a noble gas with a full octet and does not form covalent bonds under normal conditions.
Why D is wrong: D is wrong because calcium is an alkaline earth metal that primarily forms ionic bonds, not covalent chains.
MCQ 6Direct ApplicationPractice
A student claims: 'Carbon can form a maximum of four covalent bonds, so it can never form more than four bonds in any compound.' This statement is:
Show answer and why every option is right or wrong
Answer: B. B is correct. Carbon has no accessible d-orbitals in its valence shell (n = 2), so it cannot expand beyond an octet. Four bonds is the maximum. While unusual bonding situations exist in advanced chemistry (e.g., hypercoordinate carbon in certain carbocations), at the NCERT/NEET level, carbon strictly forms a maximum of four covalent bonds. NCERT Class 11 Chemistry, Chapter 8, page 257.
Why A is wrong: A is wrong because 'exactly four in every compound' is a stronger claim than tetravalency makes. In carbon monoxide the carbon has three bonds and a lone pair, so four is the CEILING, not a quota that every compound meets — which is what option B says.
Why C is wrong: C is wrong because carbon's principal quantum number is 2, meaning no 2d orbitals exist; octet expansion via d-orbitals is impossible for period-2 elements.
Why D is wrong: D is wrong because the 3s orbital is not part of carbon's valence shell. Promotion in carbon is 2s → 2p, both within n = 2; reaching n = 3 costs far more energy than the extra bonds could repay, so no such expansion happens.
MCQ 7Concept TrapPractice
Carbon in the excited state has the configuration 2s¹ 2pₓ¹ 2pᵧ¹ 2p_z¹. The four orbitals have different energies (2s < 2p), yet carbon in methane (CH₄) forms four EQUIVALENT bonds. Which concept explains this equivalence?
Show answer and why every option is right or wrong
Answer: C. C is correct. In sp³ hybridisation, one 2s and three 2p orbitals mix to form four equivalent hybrid orbitals, explaining why all four C–H bonds in methane are identical in length and energy. NCERT Class 11 Chemistry, Chapter 8, page 257.
Why A is wrong: A is wrong because resonance describes delocalisation of π-electrons across multiple structures; it does not explain the equivalence of σ-bonds formed from different-energy atomic orbitals.
Why B is wrong: B is wrong because the inductive effect describes electron displacement through σ-bonds due to electronegativity differences between bonded atoms — it has nothing to do with orbital equivalence.
Why D is wrong: D is wrong because hyperconjugation involves delocalisation of σ-electrons from C–H bonds into adjacent empty or partially filled orbitals — it does not explain why four separate C–H bonds are equivalent.
MCQ 8CalculationPractice
Consider the following statements about carbon:
(I) Ground-state carbon has four unpaired electrons.
(II) The energy of 2s→2p promotion is compensated by forming extra bonds.
(III) Carbon cannot expand its octet because n = 2 has no d-orbitals.
(IV) In the excited state, carbon has the configuration 1s² 2s¹ 2p³.
How many of the above statements are CORRECT?
Show answer and why every option is right or wrong
Answer: A. A is correct. Statement (I) is false — ground-state carbon has two unpaired electrons (2s² 2p²); four unpaired electrons appear only in the excited state. Statements (II), (III), and (IV) are all correct, giving three correct statements. NCERT Class 11 Chemistry, Chapter 8, page 257.
Why B is wrong: B is wrong because it undercounts; only statement (I) is incorrect (ground state has 2 unpaired electrons, not 4).
Why C is wrong: C is wrong because three statements — (II), (III), and (IV) — are correct, not just one.
Why D is wrong: D is wrong because statement (I) is false. Ground-state carbon (1s² 2s² 2p²) has exactly two unpaired electrons.
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How do you solve a Tetravalency Carbon question? A worked example
- 1
Given
Carbon has atomic number 6. Electronic configuration: 1s² 2s² 2p². A student is asked to explain why carbon is tetravalent despite having only two unpaired electrons in the ground state.
- 2
Required
Explain the electronic basis of carbon's tetravalency. Identify the excited-state configuration and the number of bonds carbon can form.
- 3
Concept
In the ground state, carbon's 2s orbital is fully occupied and 2p has two unpaired electrons. To form four bonds, one 2s electron must be promoted to the empty 2p orbital. The promotion energy is offset by the bond energy gained from two additional covalent bonds.
- 4
Formula / principle
No mathematical formula applies. The governing principle is: promotion of 2s¹ → 2p gives four singly occupied orbitals → four covalent bonds → tetravalency.
- 5
Substitution
Ground state: 1s² 2s² 2pₓ¹ 2pᵧ¹ 2p_z⁰ → 2 unpaired electrons.
Excited state: 1s² 2s¹ 2pₓ¹ 2pᵧ¹ 2p_z¹ → 4 unpaired electrons. - 6
Calculation
No numerical calculation. The count of unpaired electrons goes from 2 (ground) to 4 (excited). These four orbitals (one 2s + three 2p) undergo hybridisation (sp³ in saturated compounds, sp² in alkenes, sp in alkynes) to form equivalent bonding orbitals.
- 7
Final answer
Carbon is tetravalent because excitation from 2s² 2p² → 2s¹ 2p³ provides four half-filled orbitals, and the energy recovered by forming four bonds exceeds the promotion cost. This is why carbon forms four covalent bonds in virtually all its compounds.
- 8
Common trap
Students sometimes write the ground-state configuration as having four unpaired electrons. Always specify that four unpaired electrons exist only after the 2s→2p excitation step. On a NEET OMR, confusing ground and excited states leads to picking the wrong configuration option.
- 9
Similar NEET-style question
"Explain why carbon, with only two unpaired electrons in its ground state, shows a covalency of four. What role does the absence of d-orbitals in the n = 2 shell play in limiting carbon's maximum covalency?"
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What to remember before solving Tetravalency Carbon questions
More in Organic Chemistry — Some Basic Principles: 9 exam traps and mistakes · 1 formula · 1 question pattern from its other lessons.
Tetravalency Carbon questions from past NEET papers
No question in our NEET 2020–2025 set targets this topic directly.
All 12 past-paper questions from Organic Chemistry — Some Basic Principles →
Sources
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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