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NDA Chemistry · Chemical Bonding

Bond Counting and Molecular Structure

Once you know how many bonds each atom forms, you can count every covalent bond in a small molecule by adding up the C–C, C–H and C–X links — and you can spot odd-electron molecules that pair up (dimerize) to satisfy the octet.

All Chemical Bonding notesNDA Chemistry strategy

Why this matters

A small subtopic — 2 PYQs — but a reliable one. The bank gives a molecular formula and asks for the total number of covalent bonds, or names an oxide and asks which one dimerizes. Both come straight from the valency rules: carbon forms 4 bonds, hydrogen 1, oxygen 2, and an atom with an unpaired electron will pair up with a partner.

Concept 1 of 2

Counting the covalent bonds in a molecule

Intuition

Each atom forms a fixed number of bonds — carbon 4, hydrogen 1, oxygen 2, chlorine 1, nitrogen 3. To count the bonds in a molecule, just tally the links: the carbon skeleton (C–C bonds), then every C–H, C–O, O–H or C–Cl bond around it. The bank gives a formula like C₃H₇Cl and asks for the total.

Definition

How to count covalent bonds:

  • Each single bond is one shared pair = one covalent bond. (A double bond counts as the structure dictates; for these PYQs the molecules are all single-bonded.)
  • Bonds per atom: C = 4, N = 3, O = 2, H = 1, Cl = 1.
  • Build the skeleton, then add bonds: for C₃H₇Cl — C–C bonds = 2, C–H bonds = 7, C–Cl bond = 1 → 10 covalent bonds.
  • For methanol CH₃OH (CH₄O) — C–H bonds = 3, C–O bond = 1, O–H bond = 1 → 5 covalent bonds.

Bonds formed per atom

C=4N=3O=2H=1Cl=1\text{C}=4 \quad \text{N}=3 \quad \text{O}=2 \quad \text{H}=1 \quad \text{Cl}=1

Worked example

How many covalent bonds are present in a chloropropane molecule, C₃H₇Cl?
Practice this conceptself-check · 5 quick reps

From the bank · past-year question

Example 1Chemical BondingEASY
What is the total number of covalent bonds in methanol?

[Q143 · Sep · 2021]

Count every link, including the O–H in an alcohol

For methanol CH₃OH the answer is 5, not 4 — don't forget the O–H bond on top of the three C–H bonds and the C–O bond. Tally the whole skeleton, every atom included.

Concept 2 of 2

Odd-electron molecules and dimerization

Intuition

A molecule with an odd total number of valence electrons must have one unpaired electron — it is a free radical. Such molecules are reactive and often pair up with a copy of themselves (dimerize) so that the lone electrons join into a bond and both atoms reach a stable arrangement. The bank asks which nitrogen oxide dimerizes.

Definition

Dimerization of odd-electron molecules:

  • A molecule with an odd number of valence electrons has an unpaired electron — it is a paramagnetic free radical.
  • Two such molecules can join, pairing their lone electrons into a new bond — this is dimerization.
  • Nitrogen dioxide NO₂ has an unpaired electron, so it readily dimerizes to dinitrogen tetroxide N₂O₄: 2 NO₂ ⇌ N₂O₄.
  • The other common nitrogen oxides (N₂O, N₂O₃, N₂O₅) have all electrons paired and do not dimerize this way.

Worked example

Which of these nitrogen oxides may dimerize: N₂O, NO₂, N₂O₃ or N₂O₅?
Practice this conceptself-check · 4 quick reps

From the bank · past-year question

Example 2Chemical BondingMODERATE
Which one of the following nitrogen oxides may dimerize?

[Q82 · Apr · 2025]

NO₂ dimerizes because it is a radical, not N₂O or N₂O₅

Among the nitrogen oxides only NO₂ has an unpaired electron, so it is the one that dimerizes (to N₂O₄). N₂O, N₂O₃ and N₂O₅ are all even-electron and do not pair up this way.

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)

Watch out for (2)

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2 questions from the bank — paginated, with cart and Word-export support.

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