What is covalent bonding in the context of Edexcel IGCSE Chemistry?

Covalent bonding is a type of chemical bond where two atoms share one or more pairs of electrons. This type of bonding typically occurs between non-metal atoms. In the Edexcel IGCSE Chemistry curriculum, understanding covalent bonding is crucial as it explains how molecules are formed and the properties of substances like water and carbon dioxide.

How do covalent bonds differ from ionic bonds in Principles of Chemistry?

Covalent bonds differ from ionic bonds in that covalent bonds involve the sharing of electrons between atoms, while ionic bonds involve the transfer of electrons from one atom to another, resulting in the formation of charged ions. In the Edexcel IGCSE Chemistry syllabus, students learn that covalent bonds typically form between non-metals, whereas ionic bonds form between metals and non-metals.

What are some common examples of covalent compounds studied in Edexcel IGCSE Chemistry?

Common examples of covalent compounds include water (H2O), carbon dioxide (CO2), methane (CH4), and ammonia (NH3). These compounds are studied in the Edexcel IGCSE Chemistry curriculum to illustrate how covalent bonding results in the formation of molecules with specific properties and structures.

Why is the concept of covalent bonding important in understanding molecular structures?

Covalent bonding is essential for understanding molecular structures because it explains how atoms are held together in molecules. This knowledge helps students predict the shapes, angles, and properties of molecules, which are key concepts in the Edexcel IGCSE Chemistry curriculum. Understanding covalent bonding allows students to grasp how molecular geometry affects physical and chemical properties.

How does the concept of electron sharing in covalent bonding relate to molecular stability?

In covalent bonding, atoms achieve greater stability by sharing electrons to fill their outer electron shells, achieving a noble gas configuration. This electron sharing results in the formation of stable molecules. In the Edexcel IGCSE Chemistry course, students learn that this stability is a driving force behind the formation of covalent bonds and is crucial for understanding the behavior of molecules in chemical reactions.

Why is "Saying covalent bonds break when methane (or other simple molecular substances) melts" a common mistake in Covalent Bonding?

Why it happens: Confusing intramolecular (within molecule) with intermolecular (between molecules) forces. How to avoid it: Melting / boiling a simple molecular substance breaks ONLY the weak INTERMOLECULAR forces — the covalent bonds INSIDE each molecule remain intact. Use the keyword 'intermolecular forces'. Source: 4CH1 Examiner Reports 2022-2024

Why is "Saying electrons are TRANSFERRED in a covalent bond" a common mistake in Covalent Bonding?

Why it happens: Confusing covalent with ionic bonding. How to avoid it: Covalent = SHARED. Ionic = TRANSFERRED. The two are mutually exclusive at IGCSE. If you see two non-metals, expect covalent (sharing). Source: 4CH1 Examiner Reports 2022-2024

Why is "Calling buckminsterfullerene C₆₀ a giant covalent substance" a common mistake in Covalent Bonding?

Why it happens: Assuming a large molecule = giant covalent. How to avoid it: C₆₀ is a single, discrete molecule with a fixed formula. It is SIMPLE MOLECULAR (despite the large size). 'Giant' covalent means the lattice has no molecular formula — it extends indefinitely.

Why is "Drawing dot-and-cross diagrams without lone pairs" a common mistake in Covalent Bonding?

Why it happens: Treating bonds as the only feature worth drawing. How to avoid it: Always count and draw the lone pairs on every atom. O in H₂O has 2; N in NH₃ has 1; each O in CO₂ has 2. They are mark-bearing. Source: 4CH1 Examiner Reports 2022-2024

Why is "Claiming diamond conducts because it is made of carbon (like graphite)" a common mistake in Covalent Bonding?

Why it happens: Generalising from graphite's conductivity. How to avoid it: In DIAMOND, all 4 outer electrons of each C are used in covalent bonds — NO delocalised electrons → no conduction. In GRAPHITE, each C uses only 3 in bonds, so 1 is delocalised per C → conducts. Source: 4CH1 Examiner Reports 2022-2024

Why is "Describing graphite as a 3D lattice (like diamond)" a common mistake in Covalent Bonding?

Why it happens: Forgetting the layered structure. How to avoid it: Graphite is made of 2D LAYERS of hexagons. Layers are held by WEAK intermolecular forces, not covalent bonds → layers slide easily → graphite is soft.

Principles of ChemistryEdexcel IGCSE Chemistry (4CH1)

Covalent Bonding

Work through the notes, try the practice questions, then take the quiz. The report tells you exactly what to revise next.

Previous Next

Learn this Topic

Start with the slides for the quick version, then go deeper with the full study notes.

Short Study Notes in the form of Slides

Read the notes first. If the method in a worked example clicks, you're ready for the questions.

Page 1 / 0

Detailed Study Notes

Full prose, callouts and a recap — built for A* mastery, not just a quick scan.

Take these study notes with you

Download a branded PDF — full prose, callouts, recap and memorise list for Covalent Bonding, ready to print or save offline.

Covalent Bonding — Pearson Edexcel International GCSE Chemistry 4CH1 Study Notes (2026 onwards, Spec Issue 4)

How non-metal atoms SHARE electron pairs to reach noble-gas configuration. Dot-and-cross diagrams for the standard 4CH1 molecules (H₂, Cl₂, HCl, H₂O, NH₃, CH₄, O₂, N₂, CO₂). The contrast between simple molecular substances (low mp, no conduction) and giant covalent structures (diamond, graphite, graphene, silicon dioxide).

At a glance

Covalent bond = shared pair of electrons between two atoms (usually two non-metals).
Single / double / triple bonds: 1, 2 or 3 shared pairs. CO₂ has two double bonds; N₂ has a triple bond.
Each atom reaches noble-gas configuration by counting both its own and shared electrons.
Simple molecular: low mp / bp because only WEAK INTERMOLECULAR FORCES need overcoming; covalent bonds inside molecules are NOT broken.
Giant covalent (macromolecular): very high mp because strong covalent bonds throughout the lattice must be broken.
Diamond: each C bonded to 4 others (tetrahedral) → very hard, mp > 3500 °C, NO free electrons → does NOT conduct.
Graphite: each C bonded to 3 others (layers); 1 DELOCALISED electron per C → CONDUCTS; layers slide → soft, slippery.
Graphene = 1-layer graphite. C₆₀ buckminsterfullerene = simple molecular cage molecule.

What you’ll learn

Mapped to the Cambridge IGCSE 4CH1 syllabus (2026 onwards).

1.56 — Understand that covalent bonding involves the SHARING of pairs of electrons between atoms.
1.57 — Understand that a covalent bond is a strong bond formed by the electrostatic attraction between the shared pair of electrons and the nuclei of the two bonded atoms.
1.58 — Draw dot-and-cross diagrams to show single covalent bonds in H₂, Cl₂, HCl, H₂O, NH₃ and CH₄.
1.59 — Draw dot-and-cross diagrams to show double covalent bonds in O₂ and CO₂ and the triple bond in N₂.
1.62 — Explain why simple molecular compounds have low melting and boiling points in terms of weak forces between molecules.
1.65 — Explain why substances with giant covalent structures have high melting and boiling points.
1.66 — Explain how the structures of diamond, graphite and C₆₀ fullerene influence their physical properties.
1.67 — Recall the structures of diamond, graphite, graphene and C₆₀ fullerene as examples of carbon allotropes.
1.68 — Explain why graphite (and graphene) conducts electricity whereas diamond does not, in terms of delocalised electrons.
1.69 — Recall that silicon(IV) oxide (silica) has a giant covalent structure similar to that of diamond.

What is a covalent bond? (spec 1.56, 1.57)

A shared pair of electrons, one from each atom.

A covalent bond is a shared pair of electrons between two atoms. Almost always the two atoms are non-metals (a metal + non-metal gives an ionic bond instead, by electron transfer).

Why share? Each atom in the bond contributes ONE electron to the shared pair. The shared pair is attracted to BOTH nuclei by electrostatic forces, holding the atoms together. The atoms can now "count" both electrons of the shared pair toward their own outer-shell total — so they can reach a noble-gas configuration by sharing rather than by transferring electrons completely.

Electron-counting rule for each atom in a covalent bond.

$\text{outer-shell electrons} = (\text{own outer electrons not in bonds}) + 2 \times (\text{number of bonds the atom is in})$

(Every bond counts as 2 electrons for each atom in it, because the shared pair belongs to BOTH.)

Worked counting — H₂O. Oxygen has 6 outer electrons. It forms 2 single bonds (to 2 H atoms) and has 2 lone pairs (4 electrons). Total around O: 4 (lone pairs) + 2 × 2 (each O–H bond counts 2 toward O's outer shell) = 8 electrons ✓ (Ne configuration).

Strength of the bond. A covalent bond is a strong bond. The energy needed to break a typical C–C single bond is ~350 kJ/mol; C=O is ~750 kJ/mol; N≡N is ~945 kJ/mol. These are comparable to or stronger than ionic-lattice energies — and MUCH stronger than intermolecular forces (~5-20 kJ/mol).

Key distinction. Covalent = electrons SHARED (each atom keeps "its own" electron and gains access to a second). Ionic = electrons TRANSFERRED (electron leaves one atom and arrives on the other). Examiners specifically test the word choice — use SHARED for covalent.

Covalent bond = SHARED PAIR of electrons.
One electron from each atom; shared pair attracted to both nuclei.
Each atom reaches noble-gas configuration (2 for H; 8 for C, N, O, halogens).
Covalent bonds are STRONG (350-950 kJ/mol).

See the full worked example for covalent bonding →

Dot-and-cross diagrams: single bonds (spec 1.58)

H₂, Cl₂, HCl, H₂O, NH₃, CH₄.

Draw the outer-shell electrons only. Use dots (•) for one element's electrons and crosses (×) for the other's. The shared pair appears in the overlap region between the two atoms — usually shown as one dot + one cross close together.

Unlike ionic dot-and-cross, NO square brackets or charges are needed — atoms in a covalent molecule are neutral overall.

The six 4CH1 single-bond molecules.

Molecule	Outer electrons	Bonds drawn	Lone pairs
H₂	H = 1, H = 1	1 shared pair between the two H	None
Cl₂	Cl = 7, Cl = 7	1 shared pair between the two Cl	3 lone pairs on each Cl
HCl	H = 1, Cl = 7	1 shared pair between H and Cl	3 lone pairs on Cl
H₂O	O = 6, H = 1 (×2)	2 shared pairs (one O–H each)	2 lone pairs on O
NH₃	N = 5, H = 1 (×3)	3 shared pairs (one N–H each)	1 lone pair on N
CH₄	C = 4, H = 1 (×4)	4 shared pairs (one C–H each)	None on C

Pattern. For each atom: (number of bonds) + (number of lone pairs × 2) gives back the outer-shell count. For H: 1 bond, 0 lone pair. For Cl in Cl₂: 1 bond + 3 lone pairs × 2 = 7 outer electrons ✓.

Drawing tip. Always place the central atom (the one that makes the most bonds — C, N or O) in the middle, with outer atoms (H, Cl) around it. Draw lone pairs on the outer side of the central atom.

Mark-scheme requirements.

Outer shell only (don't draw 2,8,... inner shells).
Show dot + cross in each shared region to make the sharing explicit.
Include lone pairs — they are mark-bearing.
No brackets or charges (these are for ionic).

Outer shell only; dots for one element, crosses for the other.
Shared pair in overlap region (one dot + one cross).
No brackets or charges for covalent species.
Lone pairs MUST be shown for full marks.

See the full worked example for covalent bonding →

Double and triple bonds: O₂, CO₂, N₂ (spec 1.59)

Multiple shared pairs in one bond.

Some atoms need more than one electron to reach a full octet, so they share multiple pairs with the same partner:

Two shared pairs = double bond. Written with $=$ in line structures (O=O, C=O).
Three shared pairs = triple bond. Written with $\equiv$ (N≡N).

Oxygen O₂ (double bond). Each O has 6 outer electrons and needs 2 more. By sharing 2 pairs with the other O, each atom contributes 2 of its electrons to bonding and keeps 4 as 2 lone pairs.

Dot-and-cross: O(•• ••)(• × | • ×)(•• ••)O — i.e. between the two atoms, 2 dots + 2 crosses (4 shared electrons = 2 shared pairs), with 2 lone pairs on each O outside.
Each O has 8 outer electrons: 4 of its own (2 lone pairs) + 4 shared (2 bonding pairs).

Carbon dioxide CO₂ (two double bonds). C has 4 outer; each O has 6 outer.

C shares 2 pairs with each O → 2 double bonds in total: O=C=O.
C uses all 4 of its outer electrons (2 with each O). C has NO lone pairs.
Each O uses 2 of its outer electrons in bonding, keeping 4 as 2 lone pairs.
Final: C has 8 outer electrons (all shared); each O has 8 (4 own + 4 shared).

Nitrogen N₂ (triple bond). Each N has 5 outer electrons and needs 3 more. By sharing 3 pairs with the other N, each atom keeps 2 of its electrons as 1 lone pair.

Dot-and-cross: each N has 1 lone pair (2 electrons) on the outside; between the atoms, 3 dots + 3 crosses (6 shared electrons = 3 shared pairs).
Each N has 8 outer electrons: 2 (lone pair) + 6 (3 bonding pairs).

Bond strength order. Triple > double > single (more shared pairs = stronger). This is why N₂ is so unreactive — the triple bond is one of the strongest covalent bonds.

Mark-scheme tip. When the answer requires "DOUBLE bond" or "TRIPLE bond", do not write "two bonds" or "three bonds" — the exact phrase is needed. Show 2 shared pairs (4 electrons) for a double bond, 3 shared pairs (6 electrons) for a triple bond.

Double bond = 2 shared pairs (e.g. O=O, C=O).
Triple bond = 3 shared pairs (e.g. N≡N).
Always draw the correct NUMBER of shared electrons.
Lone pairs still required.

See the full worked example for covalent bonding →

Simple molecular substances (spec 1.62)

Low mp, no conduction, weak forces between molecules.

Simple molecular substances consist of small, discrete molecules. The atoms within each molecule are held together by strong covalent bonds, but the molecules themselves are held to each other only by weak intermolecular forces (sometimes called van der Waals forces).

Examples on the 4CH1 spec. H₂, O₂, N₂, Cl₂, HCl, H₂O, NH₃, CH₄, CO₂, sugars (e.g. C₆H₁₂O₆), iodine I₂, sulfur S₈, buckminsterfullerene C₆₀.

Property 1: Low melting and boiling points.

When you melt or boil a simple molecular substance, you are NOT breaking the covalent bonds inside the molecules. The molecules stay intact — you only need to overcome the weak intermolecular forces so they can separate (liquid → gas).

This is THE crucial distinction:

What's overcome on melting	Force strength	Energy needed	mp / bp
Strong covalent bonds (giant covalent)	Strong	High	Very high (>2000 °C)
Weak intermolecular forces (simple molecular)	Weak	Low	Low (often <0 °C)

Methane CH₄ boils at −162 °C; water H₂O boils at 100 °C (only this high because of hydrogen bonding — a special, stronger intermolecular force). CO₂ sublimes at −78 °C.

Property 2: No electrical conductivity.

Simple molecular substances have:

NO free electrons (all outer electrons in bonds or lone pairs).
NO ions (covalent bonding means SHARING, not transfer — no charged species).

Without mobile charge carriers, no conduction in ANY state — solid, liquid or gas.

Exception. HCl dissolved in water DOES conduct, but only because water reacts with HCl to form H⁺(aq) and Cl⁻(aq) ions — it's no longer pure simple molecular HCl.

Property 3: Solubility patterns.

Non-polar molecules (CH₄, Cl₂, I₂) are insoluble in water (polar) but soluble in non-polar solvents (petrol, hexane).
Polar molecules (HCl, NH₃, sugar) ARE soluble in water because water can form attractive dipole–dipole interactions with them.

Trend down a group / homologous series. Larger molecules have stronger intermolecular forces (more electrons for van der Waals attractions) → higher mp / bp. So I₂ is a solid at rt; Br₂ a liquid; Cl₂ a gas. Same for alkanes: butane (C₄) is a gas; petrol (~C₆-C₁₂) is a liquid; candle wax (C₂₀+) is a solid.

Small discrete molecules + weak intermolecular forces.
Low mp / bp — intermolecular forces overcome, NOT covalent bonds.
No free electrons + no ions = no conduction.
Solubility depends on polarity (water dissolves polar; non-polar in non-polar solvents).

See the full worked example for covalent bonding →

Giant covalent (macromolecular) substances (spec 1.65, 1.66)

Continuous lattice → very high mp, hard.

Giant covalent substances (also called macromolecular) are continuous lattices of atoms held together by covalent bonds throughout — there are no separate "molecules". The whole crystal is essentially one giant molecule.

Examples on the 4CH1 spec. Diamond, graphite, graphene, silicon(IV) oxide SiO₂ (silica / quartz / sand).

Property 1: Very high melting and boiling points.

To melt a giant covalent substance, MANY strong covalent bonds must be broken throughout the lattice. This requires a huge amount of energy → mp typically > 2000 °C (diamond > 3500 °C; SiO₂ ~1700 °C). All giant covalent substances are solids at room temperature.

Property 2: Hardness.

The rigid 3D (or 2D) network of strong covalent bonds means giant covalent solids resist deformation. Diamond and SiO₂ are extremely hard — used in cutting and abrasive tools. Graphite, despite being giant covalent, is SOFT because of the layered structure (covered below).

Property 3: Insolubility.

No solvent can break the continuous network of strong covalent bonds → giant covalent substances are insoluble in water and in nearly all solvents.

Property 4: Electrical conductivity — usually no.

For DIAMOND and SiO₂: all outer-shell electrons are tied up in covalent bonds, with no free / delocalised electrons → no conduction.

For GRAPHITE and GRAPHENE: yes — there is 1 delocalised electron per C atom that can move along the layers and carry charge. See next section.

Continuous lattice — no discrete molecules.
Strong covalent bonds throughout → very high mp.
Usually hard (exception: graphite is soft because of layers).
Generally insoluble; generally non-conducting (exception: graphite + graphene).

See the full worked example for covalent bonding →

Carbon allotropes: diamond, graphite, graphene, C₆₀ (spec 1.67, 1.68)

Four forms of pure carbon with very different properties.

Carbon's outer-shell of 4 electrons lets it form 4 covalent bonds. Depending on HOW those 4 bonds are arranged, very different solids result.

Diamond.

Each C atom forms 4 single covalent bonds to 4 other C atoms in a tetrahedral arrangement.
This creates a rigid, continuous 3D lattice — every atom locked into a strong network.
Hardness: hardest natural substance — used in drill bits, saw blades, cutting tools.
Melting point: > 3500 °C (sublimes; needs to break millions of strong covalent bonds).
Conductivity: NO — all 4 outer electrons in covalent bonds, no delocalised electrons.

Graphite.

Each C atom forms 3 single covalent bonds to 3 other C atoms in hexagonal rings arranged in flat LAYERS.
Each C has 1 remaining outer electron that is DELOCALISED — it moves freely between the layers.
Layers are held together by weak intermolecular forces (van der Waals), NOT covalent bonds.
Softness: layers can SLIDE over each other → soft, slippery → used as a lubricant and as pencil "lead".
Melting point: very high (covalent bonds within layers are strong).
Conductivity: YES — delocalised electrons move along the layers and carry charge. Used as electrodes (e.g. in electrolysis, batteries).

Graphene.

A single layer of graphite — one atom thick.
Each C bonded to 3 others in a 2D hexagonal sheet, with 1 delocalised electron per C.
Properties: extremely strong (one of the strongest materials known per unit weight), highly conductive (faster than copper), transparent, flexible.
Uses: flexible touchscreens, transistors, high-capacity batteries, conductive inks, composite materials.

Buckminsterfullerene C₆₀.

A discrete molecule of 60 C atoms arranged in 20 hexagons and 12 pentagons → spherical "football-shaped" cage.
Each C bonded to 3 others by single bonds (with some bond character spread over the cage).
Classification: SIMPLE MOLECULAR (despite being a big molecule), because separate C₆₀ molecules are held together by weak intermolecular forces in the solid.
Properties: relatively low mp (~280 °C — much lower than diamond); soluble in some non-polar solvents.
Uses: drug delivery (the hollow cage carries molecules), lubricants, potential medical applications.

Silicon dioxide SiO₂ (silica / quartz / sand). Each Si bonded to 4 O atoms tetrahedrally; each O bonded to 2 Si. Similar to diamond structure → high mp, hard, insulator. Used in glass-making, microelectronics (silicon chips with SiO₂ insulation layer), and as sand.

Diamond: 4 bonds/C, tetrahedral, hard, insulator.
Graphite: 3 bonds/C, layered, soft, conducts.
Graphene: single graphite layer — strong + conducting + transparent.
C₆₀ fullerene: discrete molecule, SIMPLE molecular, low-ish mp.
SiO₂: diamond-like Si-O lattice, very high mp.

See the full worked example for covalent bonding →

How it’s examined

Covalent bonding is a guaranteed Paper 1 (4CH1/1C) topic on every session. Most-common items: (1) dot-and-cross diagram (or description) of a simple molecule — 3-5 marks; (2) explain the difference in mp between a simple molecular and a giant covalent substance — 4-6 marks; (3) why graphite conducts but diamond doesn't — 4-6 marks; (4) classify a structure as simple molecular vs giant covalent and justify — 3-4 marks; (5) define 'covalent bond' — 2-3 marks. Paper 2 (4CH1/2C) sometimes asks about C₆₀ fullerene uses or compares allotropes. Worth 8-15 marks across both papers per session.

Sources: Pearson Edexcel International GCSE Chemistry 4CH1 specification — Issue 4, September 2024 (valid for 2026 onwards); Pearson Edexcel 4CH1 Examiner Reports 2022-2024; 4CH1/1C May/Jun 2024 question paper and mark scheme; 4CH1/1C January 2024 question paper and mark scheme; 4CH1/1CR January 2024 question paper and mark scheme; 4CH1/2C May/Jun 2024 question paper and mark scheme. Last reviewed 2026-05-12.

Master this Topic

Worked examples, formulae, definitions and the mistakes examiners flag — everything you need to push from a pass to an A*.

Take this whole topic with you

Download a branded revision sheet — worked examples, formulae, definitions and common mistakes for Covalent Bonding, ready to print or save as PDF.

Step-by-step worked examples — Covalent Bonding

Step-by-step solutions to past-paper-style questions on covalent bonding, written exactly the way a tutor would explain them at the board.

1Describing a covalent bond using H₂ (4 marks, Paper 1)
Core• Adapted from 4CH1/1C Jan 2024 Q4• covalent bond, dot-cross, spec-1.56, spec-1.57
Question
Hydrogen exists as diatomic H₂ molecules. Describe how a covalent bond is formed between two hydrogen atoms, referring to electron configuration. Include a description (in words) of the dot-and-cross diagram. (4 marks)
Step-by-step solution
1. Step 1
  Each H atom starts with 1 outer electron (1 M). A hydrogen atom has electron configuration $1s^1$ — just 1 electron in its only shell. It is one electron short of the stable helium configuration (2 electrons, full first shell).
2. Step 2
  Sharing a pair of electrons (1 A). When two H atoms approach, each contributes its 1 outer electron and the two electrons are SHARED in the region between the nuclei. This shared pair counts in the outer shell of BOTH atoms.
3. Step 3
  Both atoms attain He configuration (1 B). Each H atom now effectively has 2 outer-shell electrons — the same as helium, the nearest noble gas. The molecule is more stable than the two separate atoms.
4. Step 4
  Dot-and-cross diagram description (1 B). Draw the two H atoms close together. The electron of one H is shown as a dot (•) and the electron of the other H as a cross (×). The two electrons sit in the overlap region between the nuclei. So the molecule appears as H(•×)H — the dot and the cross together representing the shared pair.
Answer
Each H has 1 outer electron and is 1 short of He config. The two atoms SHARE a pair of electrons (one from each) — the shared pair lies between the nuclei. Both atoms now have 2 outer electrons (He config). Dot-and-cross: H(•×)H with the dot from one atom + the cross from the other in the overlap.
Examiner tip
Edexcel 4-mark mark scheme: 1 for 'each H has 1 outer electron'; 1 for 'pair of electrons SHARED' (the word SHARED is mark-bearing — not 'transferred', that's ionic); 1 for 'both achieve noble-gas / He configuration'; 1 for correct dot-and-cross description. Square brackets and ionic charges should NOT appear on covalent diagrams.
2Dot-and-cross diagram for CO₂ — double bonds (4 marks, Paper 1)
Core• Adapted from 4CH1/1C May/Jun 2024 Q4• double bond, CO2, spec-1.59
Question
Carbon dioxide (CO₂) contains double covalent bonds. Carbon (Z = 6) has electron configuration 2,4 and oxygen (Z = 8) has 2,6. Describe the bonding in CO₂ and explain (in words) the dot-and-cross diagram. (4 marks)
Step-by-step solution
1. Step 1
  Electron-counting for each atom (1 M). Carbon has 4 outer electrons and needs 4 more to reach Ne configuration (8 outer). Each oxygen has 6 outer electrons and needs 2 more.
2. Step 2
  Two oxygens — each shares 2 pairs with C (1 A). Two O atoms are needed: each one shares TWO pairs of electrons with C (a DOUBLE bond). So C uses all 4 of its outer electrons (2 with each O) and each O uses 2 of its outer electrons (the other 4 remain as 2 lone pairs).
3. Step 3
  Final electron count (1 B). C now has 8 outer electrons (4 of its own + 4 shared from O atoms). Each O has 8 outer electrons (6 of its own + 2 shared from C). All three atoms have noble-gas configuration (Ne).
4. Step 4
  Dot-and-cross description (1 B). Draw O=C=O linearly. Between the C and each O, show TWO shared pairs (two crosses from C overlapping with two dots from O). Each O also has TWO lone pairs (4 dots not involved in bonding) drawn on the outside. C has no lone pairs.
Answer
C has 4 outer e⁻, each O has 6. C shares 2 pairs with each O (a double bond each side). C now has 8 outer e⁻; each O now has 8 (4 from itself as 2 lone pairs + 4 shared). All atoms reach Ne configuration. Diagram: O=C=O with two shared pairs between each pair of atoms and two lone pairs on each O.
Examiner tip
Edexcel mark scheme: 1 for correct outer-electron counts; 1 for stating 'double bond' or 'two shared pairs'; 1 for both atoms reaching Ne configuration; 1 for the dot-and-cross description including lone pairs on O. Forgetting the lone pairs is the most common error — they MUST be shown.
3Comparing melting points of methane and diamond (5 marks, Paper 1)
Core• Adapted from 4CH1/1CR Jan 2024 Q5• simple molecular, giant covalent, spec-1.65
Question
Methane (CH₄) has a very low boiling point (−162 °C) but diamond (also containing only C and H type bonds — specifically C–C bonds) has an extremely high melting point (> 3500 °C). Both substances contain only covalent bonds. Explain this large difference. (5 marks)
Step-by-step solution
1. Step 1
  Identify the structure of each (1 M). Methane is a simple molecular substance — it consists of discrete CH₄ molecules. Diamond is a giant covalent (macromolecular) substance — a continuous lattice of carbon atoms.
2. Step 2
  Methane — what is overcome on melting (1 A). When methane melts (or boils), the covalent C–H bonds INSIDE each molecule are NOT broken. Only the weak intermolecular forces between separate CH₄ molecules need to be overcome.
3. Step 3
  Methane — low mp because weak forces (1 B). Weak intermolecular forces require very little energy to overcome → methane has a very low mp / bp and is a gas at room temperature.
4. Step 4
  Diamond — what is overcome on melting (1 A). In diamond every C atom is bonded by strong covalent bonds to 4 other C atoms in a continuous tetrahedral lattice. To melt diamond, MANY of these strong covalent bonds must be broken.
5. Step 5
  Diamond — high mp because strong covalent bonds break (1 B). Breaking strong covalent bonds requires a huge amount of energy → diamond has an extremely high mp (> 3500 °C).
Answer
Methane is simple molecular — only WEAK intermolecular forces between separate CH₄ molecules need overcoming (covalent bonds inside the molecules are not broken). Diamond is giant covalent — strong covalent bonds throughout the whole lattice must be broken. Strong bonds need much more energy → much higher mp.
Examiner tip
Edexcel 5-mark mark scheme: 1 for naming each structure type (simple molecular vs giant covalent); 1 for 'weak intermolecular forces' overcome for CH₄; 1 for 'NOT covalent bonds broken' in CH₄; 1 for 'strong covalent bonds' broken in diamond; 1 for linking energy required → mp. The CRUCIAL keyword is 'intermolecular forces' (or 'forces between molecules') — saying 'covalent bonds break' for methane melting loses 2 marks.
4Why graphite conducts but diamond does not (6 marks, Paper 1)
Core• Adapted from 4CH1/1C May/Jun 2024 Q8• graphite, diamond, conductivity, spec-1.67, spec-1.68
Question
Diamond and graphite are both giant covalent allotropes of carbon. Graphite conducts electricity but diamond does not. Both are very hard to melt. Explain these observations in terms of the structure and bonding in each substance. (6 marks)
Step-by-step solution
1. Step 1
  Diamond structure (1 M). Each C atom in diamond is bonded by 4 strong covalent bonds to 4 other C atoms in a TETRAHEDRAL arrangement. This forms a continuous, rigid 3D lattice with no free electrons or ions.
2. Step 2
  Graphite structure (1 M). Each C atom in graphite is bonded by 3 strong covalent bonds to 3 other C atoms in HEXAGONAL rings arranged in LAYERS. Each C has ONE remaining outer electron which is DELOCALISED between the layers.
3. Step 3
  Diamond conductivity (1 A). In diamond, ALL 4 outer electrons of every C atom are locked into covalent bonds. There are NO delocalised / free electrons → diamond CANNOT conduct electricity.
4. Step 4
  Graphite conductivity (1 A). In graphite, the 1 delocalised electron per C atom is free to move along the layers and carry charge → graphite CONDUCTS electricity (along the layers).
5. Step 5
  Both have high mp (1 B). Both substances are giant covalent — to melt either, large numbers of strong covalent bonds must be broken throughout the lattice (or between layers). This requires a great deal of energy → both have very high melting points.
6. Step 6
  Bonus — layer-sliding in graphite (1 B). Between graphite's layers there are only weak intermolecular forces (van der Waals), so the layers can slide over each other → graphite is soft and slippery (used as a lubricant and pencil lead), whereas diamond is the hardest natural substance.
Answer
Diamond: each C bonded to 4 others (tetrahedral); all outer electrons in covalent bonds → no free electrons → does not conduct. Graphite: each C bonded to 3 others (layers); 1 outer electron per C is delocalised and moves along the layers → conducts. Both giant covalent → strong covalent bonds throughout → high mp. (Graphite extra: weak forces between layers → layers slide → soft and slippery.)
Examiner tip
Edexcel 6-mark mark scheme: each ALLOTROPE'S structure (2 marks); conductivity explanation for EACH (2 marks — keyword 'delocalised electrons' for graphite); high mp explanation (1 mark); soft / hard explanation for graphite (1 mark). Examiner reports highlight that candidates often muddle which has 3 bonds vs 4. Memorise: DIAmond = 4 bonds (think 'D' for 'four' bonds; graphite has FEWER bonds).

Model Answers — Covalent Bonding

High-scoring sample answers for covalent bonding on the Pearson Edexcel IGCSE 4CH1 paper, with examiner-style notes mapping each response to the mark scheme and assessment objectives.

Question
4CH1/1C-style — covalent bond definition + Cl₂5 marks
Define a covalent bond. Describe how the bonding in chlorine (Cl₂) is formed, including a description of the dot-and-cross diagram, and explain why Cl atoms form a single rather than a double covalent bond. (5 marks)
Model answer
Definition of a covalent bond. A covalent bond is a shared pair of electrons between two atoms (almost always two non-metal atoms), with one electron contributed by each atom. The shared pair is attracted to the nuclei of BOTH atoms, holding them together.

Bonding in Cl₂. A chlorine atom (Z = 17) has electron configuration 2, 8, 7 — seven outer-shell electrons. To reach the stable argon configuration (2, 8, 8), it needs just ONE more electron.

Two Cl atoms approach. Each contributes ONE of its outer electrons to a shared pair. After bonding:

Each Cl atom has 7 of its own outer electrons (3 lone pairs + 1 used for sharing) PLUS 1 from the other Cl atom in the shared pair = 8 outer electrons total — the argon configuration.

Dot-and-cross diagram description. Draw the two Cl atoms close together. Show the outer 7 electrons of one Cl as dots (•) and the outer 7 of the other as crosses (×).

In the overlap region between the two nuclei: one dot + one cross (the shared pair).

On the outside of each Cl: 3 lone pairs (3 pairs of dots on one Cl, 3 pairs of crosses on the other) — 6 non-bonding electrons each.

Why single, not double? Each Cl atom needs to GAIN only 1 electron to reach the noble-gas configuration of argon. Sharing 1 pair (a single bond) is enough — sharing 2 pairs (a double bond) would give each Cl 9 outer electrons, which is not the stable octet.
Why this scores
Edexcel 5-mark mark scheme: 1 for the definition (shared pair, both atoms contribute); 1 for 'each Cl has 7 outer electrons / needs 1 more'; 1 for 'noble gas configuration / argon reached'; 1 for dot-and-cross description with lone pairs; 1 for explaining single bond is enough. The phrase 'shared pair of electrons' is the mark-bearing definition keyword.
Question
4CH1/2C-style — multiple small-molecule dot-and-cross6 marks
Describe the bonding in water (H₂O), ammonia (NH₃) and methane (CH₄). For each, state the number of bonds and lone pairs on the central atom, and explain (in words) the dot-and-cross diagram. (6 marks)
Model answer
General principle. Each atom shares enough pairs of electrons to reach a noble-gas configuration. H atoms reach 2 outer electrons (He); C, N, O atoms reach 8 outer electrons (Ne). Outer-electron counts: H = 1, C = 4, N = 5, O = 6.

1. Water (H₂O).

O has 6 outer electrons; needs 2 more.

2 H atoms each contribute 1 electron → O shares 1 pair with each H = 2 single bonds.

O still has 4 of its own electrons not used for bonding → 2 lone pairs on O.

Dot-and-cross: O in the centre with 6 dots (its electrons), surrounded by 2 H atoms each with 1 cross. Between each H–O pair: 1 dot + 1 cross (the shared pair). On the O atom (outside the bonds): 2 lone pairs of dots.

Result: O has 8 outer electrons; each H has 2 outer electrons.

2. Ammonia (NH₃).

N has 5 outer electrons; needs 3 more.

3 H atoms each contribute 1 electron → N shares 1 pair with each H = 3 single bonds.

N still has 2 of its own electrons not used for bonding → 1 lone pair on N.

Dot-and-cross: N in the centre with 5 dots, surrounded by 3 H atoms each with 1 cross. Three N–H shared pairs (each 1 dot + 1 cross). N has 1 lone pair (2 dots) on the outside.

Result: N has 8 outer electrons; each H has 2 outer electrons.

3. Methane (CH₄).

C has 4 outer electrons; needs 4 more.

4 H atoms each contribute 1 electron → C shares 1 pair with each H = 4 single bonds.

C has NO lone pairs (all 4 outer electrons used in bonding).

Dot-and-cross: C in the centre with 4 dots, surrounded by 4 H atoms each with 1 cross. Four C–H shared pairs (each 1 dot + 1 cross). No lone pairs anywhere on C.

Result: C has 8 outer electrons; each H has 2 outer electrons.

Summary table.

Molecule Bonds on central atom Lone pairs Total e⁻ on central atom
H₂O 2 2 8
NH₃ 3 1 8
CH₄ 4 0 8
Why this scores
Edexcel 6-mark mark scheme: 2 marks per molecule (1 for correct bond count + 1 for correct lone-pair count and dot-and-cross description). The lone pairs are essential — Edexcel papers are full of cases where a candidate has drawn correct bonds but missed lone pairs on O or N, losing easy marks.
Question
4CH1/2C-style — properties of simple molecular substances6 marks
Explain why simple molecular substances (such as CO₂, H₂O and Cl₂) generally have LOW melting points, do NOT conduct electricity, and are often insoluble in water. Refer to bonding and intermolecular forces in your answer. (6 marks)
Model answer
Structure of simple molecular substances. They consist of small, discrete molecules. Within each molecule, atoms are held together by strong covalent bonds (shared pairs of electrons). BETWEEN separate molecules, however, the only forces are weak intermolecular forces (van der Waals forces).

1. Low melting and boiling points. When you melt or boil a simple molecular substance, you are NOT breaking the strong covalent bonds within each molecule — those stay intact. You only need to overcome the weak intermolecular forces between the molecules. Because these forces are weak, only a small amount of energy is needed → low melting and boiling points. Many simple molecular substances are gases or volatile liquids at room temperature (CO₂, H₂O, Cl₂, NH₃, CH₄).

2. Do NOT conduct electricity. Simple molecular substances contain NO free charge carriers:

There are no free electrons — all outer electrons are tied up in covalent bonds or lone pairs.

There are no ions — covalent bonding involves SHARING, not transfer of electrons, so no ions form.

Without mobile charge carriers, electricity cannot flow. Even when molten (and the intermolecular forces overcome), the molecules themselves are still neutral, so molten simple molecular substances also do NOT conduct.

3. Solubility in water. Most simple molecular substances are insoluble in water because water cannot break the intramolecular covalent bonds and the molecules are not attracted strongly to water. However, polar molecules (such as HCl, NH₃ or sugar) DO dissolve in water because water can form attractive forces (dipole–dipole interactions or hydrogen bonds) with them. So solubility depends on whether the molecule is polar.

Summary. "Inside each molecule: strong covalent. Between molecules: weak intermolecular." This single sentence explains all three properties.
Why this scores
Edexcel 6-mark mark scheme: 2 for each property (1 for stating the property + 1 for the correct molecular-level explanation). Mark-bearing keywords: 'weak intermolecular forces', 'strong covalent bonds NOT broken', 'no free electrons or ions'. The mistake of saying 'low mp because covalent bonds are weak' is heavily penalised — covalent bonds inside molecules are STRONG.
Question
4CH1/2C-style — modern carbon allotropes6 marks
Buckminsterfullerene (C₆₀) and graphene are two allotropes of carbon discovered in recent decades. (a) Describe the structure of EACH. (b) For each, identify whether it is a simple molecular or a giant covalent substance and explain why. (c) Suggest ONE practical use for each, justified by a property of the structure. (6 marks)
Model answer
(a) Structures.

Buckminsterfullerene C₆₀: a hollow, spherical (football-shaped) molecule made of 60 carbon atoms arranged in a pattern of 20 hexagons and 12 pentagons. Each carbon atom is bonded to 3 others by single covalent bonds. The whole structure is one discrete molecule, often called a "buckyball".

Graphene: a SINGLE layer of graphite — a one-atom-thick sheet of carbon atoms arranged in hexagons. Each carbon is bonded to 3 others, with one delocalised electron per atom. Essentially a 2D giant covalent sheet.

(b) Classification.

Fullerene C₆₀ = simple molecular, despite being a large molecule. Why? Because it exists as discrete, separate molecules (each one a 60-atom sphere). Between molecules in the solid state there are only weak intermolecular forces. As a result, C₆₀ has a relatively low melting point and dissolves in some non-polar solvents.

Graphene = giant covalent (macromolecular). Why? Because the sheet extends indefinitely as a 2D network of covalent bonds. There is no "molecule" with a fixed molecular formula — the whole sheet is one continuous structure.

(c) Uses justified by property.

Fullerene C₆₀ — used in drug delivery: the hollow molecular cage can encapsulate a drug molecule and release it inside the body. Justification: discrete cage-shaped molecules with an interior cavity.

Graphene — used in flexible electronics, transparent conducting films, or as a high-strength composite. Justification: it has 1 delocalised electron per carbon (high conductivity, like graphite), is extremely strong (each C bonded to 3 others in a robust 2D lattice), and is thin enough to be transparent.

(Other valid uses: fullerenes as lubricants or catalysts; graphene in transistors, solar cells, batteries.)
Why this scores
Edexcel 6-mark mark scheme: 1 for each structure (2); 1 for each classification with justification (2); 1 for each use linked to a property (2). The classification of C₆₀ as 'simple molecular despite being a large molecule' is a common 4CH1 trick — many candidates wrongly call it giant covalent because of size. The KEY is whether it exists as discrete molecules or a continuous lattice.

Molecule	Bonds on central atom	Lone pairs	Total e⁻ on central atom
H₂O	2	2	8
NH₃	3	1	8
CH₄	4	0	8

Key Definitions and Keywords — Covalent Bonding

Definitions to memorise and the exact keywords mark schemes credit for covalent bonding answers — sharpened from recent examiner reports for the 2026 Pearson Edexcel IGCSE 4CH1 sitting.

Covalent bond
Examiner keyword
A bond formed by the sharing of a pair of electrons between two atoms (usually two non-metals). One electron in the shared pair is contributed by each atom. The shared pair is attracted to both nuclei.
Single covalent bond
Examiner keyword
A covalent bond involving ONE shared pair of electrons (2 electrons total). Examples: H–H, Cl–Cl, H–Cl, C–H, O–H.
Double covalent bond
Examiner keyword
A covalent bond involving TWO shared pairs of electrons (4 electrons total). Examples: O=O in O₂, O=C=O in CO₂, C=C in ethene.
Triple covalent bond
Examiner keyword
A covalent bond involving THREE shared pairs of electrons (6 electrons total). Example: N≡N in N₂. The strongest type of covalent bond between two atoms.
Lone pair
Examiner keyword
A pair of outer-shell electrons that is NOT involved in bonding — it belongs entirely to one atom. Example: O in H₂O has 2 lone pairs; N in NH₃ has 1 lone pair.
Simple molecular substance
Examiner keyword
A substance made of small, discrete molecules held together by weak intermolecular forces. Low mp / bp, do not conduct electricity, often insoluble in water. Examples: H₂O, CO₂, CH₄, Cl₂, O₂, NH₃, sugars, C₆₀.
Giant covalent (macromolecular) substance
Examiner keyword
A substance consisting of a continuous network of atoms bonded by strong covalent bonds throughout. Very high mp, hard, mostly insoluble, usually non-conductors (graphite/graphene are exceptions). Examples: diamond, graphite, graphene, silicon dioxide.
Intermolecular forces
Examiner keyword
Weak forces of attraction BETWEEN separate molecules (e.g. between two H₂O molecules), not within them. Much weaker than the covalent bonds INSIDE molecules. Determine the mp / bp of simple molecular substances.
Allotrope
Examiner keyword
Different physical forms of the SAME element with different arrangements of atoms. Carbon allotropes include diamond, graphite, graphene and fullerenes (e.g. C₆₀).
Delocalised electron
Examiner keyword
An electron that is NOT bound to a single atom or fixed bond — free to move within a structure. Present in graphite, graphene and metals. Responsible for electrical conductivity.

Common Mistakes and Misconceptions — Covalent Bonding

The traps other students keep falling into on covalent bonding questions — taken from recent Pearson Edexcel IGCSE 4CH1 examiner reports and mark schemes — and how to avoid them.

✕Saying covalent bonds break when methane (or other simple molecular substances) melts
4CH1 Examiner Reports 2022-2024
Why it happens
Confusing intramolecular (within molecule) with intermolecular (between molecules) forces.
How to avoid it
Melting / boiling a simple molecular substance breaks ONLY the weak INTERMOLECULAR forces — the covalent bonds INSIDE each molecule remain intact. Use the keyword 'intermolecular forces'.
✕Saying electrons are TRANSFERRED in a covalent bond
4CH1 Examiner Reports 2022-2024
Why it happens
Confusing covalent with ionic bonding.
How to avoid it
Covalent = SHARED. Ionic = TRANSFERRED. The two are mutually exclusive at IGCSE. If you see two non-metals, expect covalent (sharing).
✕Calling buckminsterfullerene C₆₀ a giant covalent substance
Why it happens
Assuming a large molecule = giant covalent.
How to avoid it
C₆₀ is a single, discrete molecule with a fixed formula. It is SIMPLE MOLECULAR (despite the large size). 'Giant' covalent means the lattice has no molecular formula — it extends indefinitely.
✕Drawing dot-and-cross diagrams without lone pairs
4CH1 Examiner Reports 2022-2024
Why it happens
Treating bonds as the only feature worth drawing.
How to avoid it
Always count and draw the lone pairs on every atom. O in H₂O has 2; N in NH₃ has 1; each O in CO₂ has 2. They are mark-bearing.
✕Claiming diamond conducts because it is made of carbon (like graphite)
4CH1 Examiner Reports 2022-2024
Why it happens
Generalising from graphite's conductivity.
How to avoid it
In DIAMOND, all 4 outer electrons of each C are used in covalent bonds — NO delocalised electrons → no conduction. In GRAPHITE, each C uses only 3 in bonds, so 1 is delocalised per C → conducts.
✕Describing graphite as a 3D lattice (like diamond)
Why it happens
Forgetting the layered structure.
How to avoid it
Graphite is made of 2D LAYERS of hexagons. Layers are held by WEAK intermolecular forces, not covalent bonds → layers slide easily → graphite is soft.

Practice questions

Exam-style questions with step-by-step worked solutions. Try one before checking the method.

Past paper style quiz

Get a report showing which sub-topics you've nailed and which ones still need work.

Video lesson

Short walkthrough of the concepts students most often get stuck on.

Covalent Bonding — frequently asked questions

The things students keep getting wrong in this sub-topic, answered.

Covalent Bonding

Short Study Notes in the form of Slides

Detailed Study Notes

What you’ll learn

How it’s examined

1Describing a covalent bond using H₂ (4 marks, Paper 1)

2Dot-and-cross diagram for CO₂ — double bonds (4 marks, Paper 1)

3Comparing melting points of methane and diamond (5 marks, Paper 1)

4Why graphite conducts but diamond does not (6 marks, Paper 1)

Covalent bond

Single covalent bond

Double covalent bond

Triple covalent bond

Lone pair

Simple molecular substance

Giant covalent (macromolecular) substance

Intermolecular forces

Allotrope

Delocalised electron

✕Saying covalent bonds break when methane (or other simple molecular substances) melts

✕Saying electrons are TRANSFERRED in a covalent bond

✕Calling buckminsterfullerene C₆₀ a giant covalent substance

✕Drawing dot-and-cross diagrams without lone pairs

✕Claiming diamond conducts because it is made of carbon (like graphite)

✕Describing graphite as a 3D lattice (like diamond)

Practice questions

Past paper style quiz

Assess your understanding

Video lesson

Covalent Bonding — frequently asked questions