Covalent Bonding

SPEC 4.2.1.4 COVALENT

Shared pairs of electrons between non-metal atoms. Dot-and-cross diagrams for the eight spec molecules — H₂, Cl₂, O₂, N₂, HCl, H₂O, NH₃, CH₄ — and the three errors AQA catches every single year.

What Is a Covalent Bond?

A covalent bond forms between two non-metal atoms. Both atoms share electrons — nothing is transferred, no ions form. Each shared pair of electrons sits between the two atoms and is attracted to both nuclei at once. That attraction is the bond.

Covalent bond

A shared pair of electrons between two atoms.

The principle

Each atom contributes one electron to a shared pair. By both counting that pair in their outer shell, both atoms achieve the electron arrangement of a noble gas:

Hydrogen (group 1, but a non-metal) ends with 2 outer electrons (full first shell, like helium)

C, N, O, halogens end with 8 outer electrons (full second/third shell, like neon/argon)

Three verbs — one warning

Ionic = transferred. Covalent = shared. Metallic = delocalised. Mixing these up is the most penalised conceptual error in Module 2 (P1 Jun24 Q05.1). If a question is about covalent bonding, the verb you want is "shared".

Three Covalent Structure Types

Covalent bonds appear in three very different arrangements. The bonds themselves are always strong — what differs is how many atoms are joined together.

1. Small molecules

Few atoms per molecule joined by covalent bonds.

Discrete units — the molecule has a definite size.

Examples: H₂, O₂, Cl₂, N₂, HCl, H₂O, CO₂, CH₄, NH₃

2. Polymers

Very large molecules — many repeating units (monomers) joined by covalent bonds.

Long chains, but still discrete molecules with ends.

Examples: poly(ethene), poly(propene), DNA, proteins

3. Giant covalent

Many atoms bonded covalently in a continuous 3D network.

No individual molecules — the whole crystal is one structure.

Examples: diamond, silicon dioxide (SiO₂), graphite

Spec recognition skill

You must be able to recognise which structure type a substance has from its formula or a given diagram. Properties of each type (mp, conductivity) are covered in Lesson 2.06; carbon-specific structures (diamond, graphite, graphene, fullerenes) are covered in Lesson 2.08.

The Eight Spec Molecules

Spec 4.2.1.4 lists eight specific molecules you must be able to draw dot-and-cross diagrams for. Memorise the bond count for each.

MoleculeFormulaBond countBond type

HydrogenH₂1 bondSingle

ChlorineCl₂1 bondSingle

Hydrogen chlorideHCl1 bondSingle

WaterH₂O2 bondsSingle (× 2)

AmmoniaNH₃3 bondsSingle (× 3)

MethaneCH₄4 bondsSingle (× 4)

OxygenO₂2 bonds (1 pair of atoms)Double (O=O)

NitrogenN₂3 bonds (1 pair of atoms)Triple (N N)

The number-of-bonds rule

Each non-metal atom forms a number of covalent bonds equal to (8 − group number). So Group 7 (Cl, F) makes 1 bond; Group 6 (O, S) makes 2; Group 5 (N, P) makes 3; Group 4 (C) makes 4. Hydrogen is the exception — it makes 1 bond (it only needs 2 outer electrons).

Dot-and-Cross Diagram Rules

Five conventions. Get any one wrong and you lose marks — AQA's mark schemes are very specific about each.

#RuleWhy it matters

1Show only the outer-shell electronsSpec convention; AQA mark schemes accept "outer shells only"

2Use dots for one atom's electrons; crosses for the other'sShows the origin of each electron in the shared pair

3The shared pair sits in the overlap region between the two atoms"Bonded pair of electrons in the overlap" (P1 Jun23 Q04.4 MS)

4Each H ends with 2 outer electrons ; each C/N/O/halogen ends with 8Adding extra electrons to H is the most common AQA-flagged error

5No brackets, no charges — covalent molecules are neutralBrackets + charges = ionic notation; covalent molecules are not ions

AQA accepts any electron symbol

Mark schemes state: "allow any combination of x, , o, e⁽⁻⁾ for electrons". The convention is two different symbols (one per atom), but the symbols themselves are flexible. Just be consistent — one symbol per atom, applied throughout.

01Worked Example: HCl 2 MARKS

Draw a dot-and-cross diagram for a molecule of hydrogen chloride (HCl). Show the outer-shell electrons only.

(Real exam question: AQA 8462/1H Jun 2023, Q04.4)

M1

Bonded pair of electrons in the overlap (1 dot from H + 1 cross from Cl, drawn in the region where the H circle and Cl circle overlap).

M2

Chlorine with 6 non-bonded electrons (3 lone pairs of crosses around the rest of the Cl outer shell).

Mark-scheme exact wording (Jun23 MS): "do not accept if extra electrons on H". Hydrogen must end with exactly 2 outer electrons (the shared pair). Don't add any non-bonded electrons to H. Final electron count: H = 2 outer (1 shared pair). Cl = 8 outer (1 shared pair + 6 non-bonded).

02Worked Example: CH₄ 2 MARKS

Draw a dot-and-cross diagram for a molecule of methane (CH₄). Show the outer-shell electrons only.

M1

Correct shared pairs: four shared pairs — one between C and each H, each shown as 1 dot (from C) + 1 cross (from H) in the overlap region.

M2

Correct electron count on each atom: C ends with 8 outer electrons (4 shared pairs); each H ends with 2 outer electrons (1 shared pair).

The check that catches errors: count the electrons around each atom in the finished diagram. C should have exactly 8 (4 dots + 4 crosses). Each H should have exactly 2 (1 dot + 1 cross). Anything else loses M2.

Reference Grid: Four More Spec Molecules

Same five rules, applied to four more spec molecules. Each one ends with H = 2 and the central non-metal = 8.

The pattern

The number of shared pairs around the central atom equals the number of bonds it forms (8 − group number). Any leftover outer electrons sit on the central atom as lone pairs: O has 2 lone pairs; N has 1 lone pair; C has none (all 4 outer electrons are in bonds in CH₄).

Double Bonds: O₂ and CO₂

When two atoms need to share more than one pair to fill their outer shells, they form a double bond (or rarely, a triple bond). A double bond is two shared pairs — four electrons in the overlap.

CO₂ — Carbon dioxide (2 double bonds)

Counting electrons

In O₂: each O has 2 + 4 + 2 = 8 outer (4 in shared pairs + 4 in lone pairs). In CO₂: C has 4 + 4 = 8 outer (no lone pairs — all in bonds); each O has 4 + 4 = 8. Always finish by checking electron counts.

Triple Bond: N₂

Nitrogen (Group 5) needs 3 bonds to fill its outer shell. In N₂, two nitrogen atoms share three pairs — a triple bond. This is one of the eight molecules the spec requires you to know — for both Foundation and Higher tier.

Electron count check

Each N has: 6 in the triple bond + 2 in its lone pair = 8 outer electrons ✓. Each N is in Group 5, so each contributes 5 outer electrons; together that's 10. Of those, 6 go into the triple bond (shared, but each N "counts" all 6) and 4 stay as 2 lone pairs (1 per N).

Why N₂ is so unreactive

The triple bond in N₂ is one of the strongest covalent bonds in chemistry — it takes a huge amount of energy to break. This is why N₂ is so stable in the atmosphere (78% of air) and why making ammonia from N₂ requires high temperature and pressure (Module 6 — Haber process).

Displayed Formulae: Lines = Shared Pairs

The spec also requires you to convert between dot-and-cross and displayed formulae, where covalent bonds are shown as lines. The rule is exact: one line = one shared pair = one covalent bond.

MoleculeShared pairs in dot-crossDisplayed formula

H₂1 shared pair (single)H—H

HCl1 shared pair (single)H—Cl

H₂O2 shared pairs (2 singles)H—O—H

CH₄4 shared pairs (4 singles)4 lines from C, one to each H

O₂2 shared pairs (1 double)O=O

CO₂4 shared pairs (2 doubles)O=C=O

N₂3 shared pairs (1 triple)N N

Poly(ethene) repeat unit5 single bonds per unit (1 C—C plus 4 C—H)−(CH₂—CH₂)ₙ− brackets enclose the repeat unit; n = a large number

Deducing molecular formula from a diagram (spec skill)

Given a dot-and-cross or displayed formula diagram, you can count atoms to find the molecular formula. Real exam example (P1 Jun21 Q01.4 propanone): the diagram shows 3 C, 6 H, 1 O → molecular formula C₃H₆O. Always count each atom type in turn and write the formula in standard order (C, then H, then other elements alphabetically).

Common error (P1 Nov21 Q01.3)

Students drew double bonds throughout CH₄ — failed. CH₄ has four single bonds (4 shared pairs, 1 between C and each H). Count the shared pairs in the dot-and-cross first, then draw the right number of lines. Count first, draw second.

Three Errors AQA Catches Every Year

Three specific errors flagged in P1 Jun22 Q04.4 and P1 Nov21 Q01.3. Each one loses easy marks.

Error 1 — Extra electrons added (P1 Jun22 Q04.4)

Adding an extra electron to H or C. H must end with exactly 2 outer electrons (1 shared pair). C must end with exactly 8 (in CH₄, four shared pairs). Drawing extra dots/crosses gives wrong totals and loses M2. Mark scheme says: "do not accept if extra electrons on H".

Error 2 — Ions drawn instead of molecule (P1 Jun22 Q04.4)

Drawing the molecule with square brackets and charges — that's ionic notation. Covalent molecules are neutral: no brackets, no charges. The atoms haven't transferred electrons; they share them.

Error 3 — Wrong double bonds (P1 Nov21 Q01.3)

Drawing double bonds where they don't belong. CH₄ has 4 single bonds, not double bonds. H₂O has 2 single bonds. Only put double bonds where the structure genuinely has them: O₂ (O=O), CO₂ (O=C=O), C=C in alkenes (Module 7).

Weak CH₄

"4 H atoms each with 2 crosses; C has 8 dots. Each C—H written as a double line."

Extra electrons on H, double bonds where there shouldn't be any.

Strong CH₄

"4 H atoms each with 1 cross. C has 4 dots. Each C—H bond shows 1 dot + 1 cross in overlap. C ends with 8 outer electrons; each H ends with 2."

Correct electron counts; correct bond multiplicity.

Limitations of Each Representation

Spec 4.2.1.4 explicitly requires you to describe the limitations of using dot-and-cross, ball-and-stick, and 2D/3D diagrams to represent molecules or giant structures. No single diagram tells the whole story.

Diagram typeShowsLimitation

Dot-and-crossWhere each electron came from; shared pairs and lone pairs; bond countDoesn't show 3D shape ; doesn't show real bond lengths or angles; suggests electrons "belong" to specific atoms when they don't

Ball-and-stick3D arrangement of atoms; bond angles; relative positionsAtoms aren't really "balls"; sticks aren't really discrete; ratio of stick length to ball size is exaggerated; doesn't show electrons

2D diagram (displayed formula)Bonds and atoms; bond multiplicity (single/double/triple)Doesn't show 3D shape; doesn't show electrons or lone pairs

3D space-fillingRealistic atom sizes; shape of the molecule; how atoms occupy spaceBonds are hidden inside; can't see electrons; harder to read bond connectivity

The principle

Each diagram type sacrifices some information to show others. To answer a "limitations" exam question well, name specifically what each diagram fails to show — don't write generic statements like "they're not realistic".

Common Errors

Students often writeCorrect versionWhy it matters

"Electrons are transferred in covalent bonding""Electrons are shared in covalent bonding"Transferred = ionic; shared = covalent (P1 Jun24 Q05.1)

"Covalent bonds form between metals""Covalent bonds form between non-metal atoms"Metal + non-metal = ionic; only non-metals make covalent bonds

Extra electron drawn on H (3 outer)Each H ends with exactly 2 outer electrons"Do not accept if extra electrons on H" (Jun23 MS)

Brackets and charges around atoms in covalent dot-crossNo brackets, no charges — molecules are neutralBrackets + charges = ionic notation (P1 Jun22 Q04.4)

Double bonds drawn between every pair of atoms in CH₄CH₄ has 4 single bonds; only draw double bonds where structure has them (O=O, C=O)Each line = 1 shared pair (P1 Nov21 Q01.3)

"Polymer is a giant covalent structure""Polymer is a large molecule "Polymers are discrete (large) molecules, not continuous networks (P1 Jun23 Q04.5)

Practice Question 1

Draw a dot-and-cross diagram for a molecule of water (H₂O). Show only the outer-shell electrons.2 MARKS

M1

Two shared pairs: central O with one shared pair (1 dot from O + 1 cross from H) to each of the two H atoms, drawn in the overlap regions.

M2

Correct electron count: O has 8 outer electrons (2 shared pairs + 2 lone pairs of dots = 4 + 4 = 8). Each H has 2 outer electrons (1 shared pair = 1 dot + 1 cross).

Common slips: forgetting to draw the 2 lone pairs on O (then O only has 4 outer electrons — wrong); putting an extra electron on a H atom; drawing double bonds (water has 2 single bonds, not a double bond).

Practice Question 2

(a) A dot-and-cross diagram for ammonia (NH₃) shows three shared pairs around N and one lone pair. Draw the displayed formula for NH₃. 1 MARK

(b) Explain why CO₂ is drawn as O=C=O rather than O—C—O. 2 MARKS

(a)

Three lines from N, one to each H. Layout: H—N(—H)—H — one line per shared pair. Lone pair on N is not drawn in displayed formula. Each line = 1 shared pair.

(b)

Mark 1: each C—O bond is a double bond (two shared pairs of electrons). Mark 2: a single line would only represent one shared pair, but the dot-and-cross shows two pairs in each overlap, so two lines (=) are needed.

The rule (P1 Nov21 Q01.3 MS):"each line represents one shared pair of electrons". Count the shared pairs in the dot-and-cross first; that's the number of lines you draw.

Practice Question 3

For each substance below, identify whether it is a small molecule , a polymer , or a giant covalent structure .4 MARKS

1

Carbon dioxide (CO₂)

2

Diamond (carbon)

3

Poly(ethene)

4

Silicon dioxide (SiO₂)

1

CO₂ = small molecule. Three atoms per molecule, discrete units.

2

Diamond = giant covalent structure. Continuous 3D network of C atoms; no molecules.

3

Poly(ethene) = polymer (a very large molecule). NOT a giant covalent structure — the chain has ends. (P1 Jun23 Q04.5 examiner alert: this is a frequently penalised slip.)

4

SiO₂ = giant covalent structure. Each Si bonded to 4 O; each O bonded to 2 Si. Continuous 3D network like diamond.

Summary

Definition

A covalent bond is a shared pair of electrons between two non-metal atoms. Both atoms count the shared pair in their outer shell — both reach a full outer shell.

The 8 spec molecules

Single bonds: H₂, Cl₂, HCl (1 bond) H₂O (2 bonds) NH₃ (3 bonds) CH₄ (4 bonds). Double: O₂ (O=O), CO₂ (O=C=O). Triple: N₂ (N N).

Dot-and-cross technique

Outer shell only; dots for one atom, crosses for the other

Shared pair in the overlap region between atoms

H ends with 2; C/N/O/halogens end with 8

No brackets, no charges — covalent molecules are neutral

Three covalent structure types

Small molecules (e.g. H₂O, CO₂) polymers (e.g. poly(ethene) — large molecules, not giant covalent) giant covalent (e.g. diamond, SiO₂ — continuous 3D networks).

Examiner alerts

Three errors AQA catches every year: (1) extra electrons on H, (2) ions drawn instead of molecules (brackets + charges), (3) wrong double bonds in molecules like CH₄. Each line in a displayed formula = one shared pair.

Ready for 2.04

Next up: Metallic Bonding — delocalised electrons, conductivity, malleability, and alloy hardness.