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NDA Chemistry · Carbon and Its Compounds

Tetra-valency, Catenation and Isomerism

Carbon forms four covalent bonds and links to itself in chains and rings, so a handful of atoms can be arranged many different ways — which is why carbon has more compounds than any other element.

All Carbon and Its Compounds notesNDA Chemistry strategy

Why this matters

The foundation of the whole chapter. The bank tests two ideas directly — the properties that make carbon special (tetra-valency + catenation), and counting the structural isomers of a small alkane. Get these two right and every later allotrope and functional-group fact has a frame to hang on.

Concept 1 of 2

Why carbon forms so many compounds

Intuition

Carbon has 4 electrons in its outer shell, so it shares 4 electrons to complete its octet — it forms exactly four covalent bonds (tetra-valency). It is also small, so those bonds are strong, and it bonds happily to other carbons — chains, branches and rings of any length (catenation). Four strong bonds plus self-linking is the whole reason organic chemistry exists.

Definition

The two defining properties and what follows from them:

  • Tetra-valency — carbon (atomic number 6, configuration 2,4) shares 4 electrons to form four covalent bonds.
  • Catenation — carbon atoms bond to one another in long chains, branches and rings; no other element does this to the same extent.
  • Carbon forms single, double and triple bonds with itself — but never a quadruple (four) bond.
  • Bonds are covalent, so most carbon compounds are poor conductors of electricity, have low melting points and are often volatile.

Worked example

Carbon has the electron configuration 2,4. Explain how many bonds it forms and why it cannot form a positive or negative ion easily.
  1. The outer shell holds 4 electrons and needs 8 for a stable octet.
  2. Losing 4 electrons (to become C4+) or gaining 4 (to become C4-) both need too much energy.
  3. So carbon instead SHARES its 4 electrons, forming four covalent bonds.
Answer:Carbon forms four covalent bonds (tetra-valency); ionising it four times over is energetically impossible, so it bonds by sharing.
Practice this concept4 quick reps

Practice — Level 1 (4 reps)

Quick reps to lock in the method. Try each, then check.

  1. 1.
    How many covalent bonds does a carbon atom form?
  2. 2.
    What is the name for carbon's ability to bond to other carbon atoms in chains and rings?
  3. 3.
    What is the maximum bond order between two carbon atoms?
  4. 4.
    Are most carbon compounds good or poor conductors of electricity?

From the bank · past-year question

Example 1Carbon and Its CompoundsMODERATE
Which among the following statements with respect to carbon is/are correct? 1. Carbon forms the basis for all living organisms and many things we use 2. Carbon shows tetra-valency and the property of catenation 3. Carbon forms covalent bonds with itself and other elements 4. Carbon forms compounds containing triple and tetra bonds between carbon atoms Select the correct answer using the code given below:

[Q117 · Sep · 2023]

Carbon forms a triple bond, not a 'four' bond

A statement that 'carbon forms compounds with quadruple bonds between carbon atoms' is false. The maximum is a triple bond (as in ethyne, HC≡CH).

Covalent means poor conductor

'Most carbon compounds are good conductors of electricity' is NOT correct — covalent compounds have no free charges. (Graphite is the famous exception, because of its delocalised electrons.)
Drill 2 more on why carbon forms so many compounds

Concept 2 of 2

Structural isomerism and counting isomers

Intuition

The same molecular formula can be built in more than one way — straight chain, branched chain, or a different branch position. These are structural isomers: same atoms, different skeletons, different compounds. For the small alkanes the bank simply asks 'how many isomers?', so learn the running count.

Definition

Structural isomers share a molecular formula but differ in the arrangement of atoms. The number of chain isomers of the straight-chain alkanes:

  • C4H10 (butane) → 2 isomers (n-butane, isobutane).
  • C5H12 (pentane) → 3 isomers (n-pentane, isopentane, neopentane).
  • C6H14 (hexane) → 5 isomers.

Methane, ethane and propane have no chain isomers (only one possible skeleton).

Structural isomer counts (alkanes)

C4H10 ⁣:2C5H12 ⁣:3C6H14 ⁣:5\text{C}_4\text{H}_{10}\!: 2 \qquad \text{C}_5\text{H}_{12}\!: 3 \qquad \text{C}_6\text{H}_{14}\!: 5

Worked example

How many structural isomers does butane (C4H10) have? Name them.
  1. Draw the straight chain of 4 carbons: n-butane.
  2. Move one carbon to a branch: a 3-carbon chain with a methyl branch on the middle carbon — isobutane (2-methylpropane).
  3. No further distinct skeleton is possible for 4 carbons.
Answer:Two structural isomers: n-butane and isobutane (2-methylpropane).
Practice this conceptself-check · 3 quick reps

Try it yourself

How many structural isomers does hexane (C6H14) have?

Practice — Level 1 (3 reps)

Quick reps to lock in the method. Try each, then check.

  1. 1.
    Number of structural isomers of pentane (C5H12)?
  2. 2.
    Number of structural isomers of propane (C3H8)?
  3. 3.
    Number of structural isomers of hexane (C6H14)?

From the bank · past-year question

Example 2Carbon and Its CompoundsMODERATE
The number of structural isomers of pentane is

[Q107 · Sep · 2022]

Pentane has 3 isomers, not 5

Students confuse the number of carbons with the number of isomers. C5H12 has 3 isomers (the carbon count is 5, the isomer count is 3). Hexane (6 carbons) is the one with 5 isomers.

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (1)

  • Structural isomerism and counting isomers

    Structural isomer counts (alkanes)

    C4H10 ⁣:2C5H12 ⁣:3C6H14 ⁣:5\text{C}_4\text{H}_{10}\!: 2 \qquad \text{C}_5\text{H}_{12}\!: 3 \qquad \text{C}_6\text{H}_{14}\!: 5

Watch out for (3)

Mastery check — 2 interleaved questions

Try each one before clicking. Questions are interleaved across the concepts above, not grouped — interleaving sharpens transfer.

Example 1Carbon and Its CompoundsMODERATE
Consider the following statements: I. Soaps are sodium or potassium salts of long-chain carboxylic acids. II. Catenation and isomerism in carbon explain the large number of organic compounds. III. Carbon dioxide is acidic but carbon monoxide is a neutral molecule. Which are correct?

[Q88 · Apr · 2026]

Example 2Carbon and Its CompoundsEASY
Which one of the following statements is not\textbf{\text{not}} correct?

[Q90 · Apr · 2022]

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