Tetravalency Carbon

8 MCQs9-step worked example
Source: NCERT Alcohols, Phenols and EthersPYQ coverage: NEET 2021, 2022, 2023, 2024, 2025Official key: NTA-verifiedLast reviewed: May 2026

Lesson

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 9 (Organic Chemistry — Some Basic Principles), page 2).

What tetravalency enables:

  1. 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.

  2. 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.

  3. 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.

Practice 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?

MCQ 2Easy RecallPractice

Carbon is tetravalent because in the excited state its electronic configuration becomes:

MCQ 3Easy RecallPractice

Which property of carbon is a DIRECT consequence of its tetravalency and small atomic size?

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:

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?

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:

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?

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) Tetravalency of carbon is the basis of catenation. How many of the above statements are CORRECT?

Worked Example

  1. 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. 2

    Required

    Explain the electronic basis of carbon's tetravalency. Identify the excited-state configuration and the number of bonds carbon can form.

  3. 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. 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. 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. 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. 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. 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. 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?" ---

Before solving, remember these

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, p. 2

Formulas

Carbocation stability order

Stability from hyperconjugation (more α-H) and inductive donation (alkyl groups). Resonance can elevate primary cations.

SymbolQuantitySI Unit
stabilityrelative-

Valid when

  • Gas phase or aprotic solvent
  • Compare similar reaction conditions

Exam Traps & Common Mistakes

These are the exact patterns that cause wrong answers in NEET. Each trap includes when it triggers and how to avoid it.

Trap

Carbocation stability order

Category: Similar Terms

Student writes 1° > 2° > 3° (linear with substitution count, but inverted) or treats methyl as more stable.

When it triggers

Question gives multiple carbocations and asks for stability ranking.

How to avoid

Stability: 3° > 2° > 1° > methyl. Hyperconjugation (more α-H = more stable). Resonance can elevate (allyl, benzyl > 1°).

Trap

Inductive vs resonance / mesomeric effect

Category: Similar Terms

Student treats inductive (through sigma bonds, decreases with distance) like resonance (through pi system, often dominant).

When it triggers

Comparison of substituent effects (acidity, basicity, dipole moment).

How to avoid

Inductive: through-bond, weakens with distance, only sigma. Resonance: through-pi-system, often more powerful, requires conjugation.

Trap

IUPAC nomenclature lowest-locant rule

Category: Similar Terms

Student numbers carbon chain from wrong end, giving higher locants to substituents than necessary.

When it triggers

IUPAC naming question with multiple substituents.

How to avoid

Choose end that gives the LOWEST set of locants for all substituents (compare set, not first-encountered). Functional group has priority for lowest locant.

Trap

Optical vs geometrical isomerism confusion

Category: Similar Terms

Student conflates optical (chirality, R/S) with geometrical (cis/trans). They're different stereoisomerism types.

When it triggers

Question about isomerism of a specific compound.

How to avoid

Optical isomerism requires chiral center (sp³ with 4 different groups). Geometrical isomerism requires restricted rotation (C=C with two different groups on each carbon).

Past Year Questions

11 questions from NEET 2021, 2022, 2023, 2024, 2025. Answers verified against NTA official keys.

NEET 2024Revised key

Identify the correct answer.

1Three resonance structures can be drawn for ozone
2BF has non-zero dipole moment 3
3Dipole moment of NF is greater than that of NH 3 3
4Three canonical forms can be drawn for CO2− ion 3
NTA Answer: Option 4(revised_final)
NEET 2022

The incorrect statement regarding chirality is

1A racemic mixture shows zero optical rotation
2S 1 reaction yields 1 : 1 mixture of both enantiomers N
3The product obtained by S 2 reaction of haloalkane having chirality at the reactive site shows inversion N of configuration
4Enantiomers are superimposable mirror images on each other
NTA Answer: Option 4(final)

How NEET usually asks this

Recurring question shapes from past papers. Each pattern shows why wrong options look tempting.

Test yourself on this topic with real past-paper questions:

Practice this topic →

Free NEET study resources

Get a structured 30-day study plan and a complete formula booklet — delivered to your inbox instantly.