What is a covalent bond and how does it form in molecules?

A covalent bond is a type of chemical bond where two atoms share one or more pairs of electrons. This bond forms when atoms, typically non-metals, have similar electronegativities and need to achieve a stable electron configuration. In a molecule, covalent bonds hold the atoms together, allowing them to share electrons and fill their outer electron shells, achieving stability similar to noble gases.

How do covalent bonds differ from ionic bonds in terms of electron sharing?

Covalent bonds involve the sharing of electron pairs between atoms, whereas ionic bonds result from the transfer of electrons from one atom to another, leading to the formation of charged ions. In covalent bonding, the shared electrons allow both atoms to attain a stable electron configuration, while ionic bonding involves the attraction between positively and negatively charged ions.

What are some examples of molecules formed by covalent bonds?

Common examples of molecules formed by covalent bonds include water (H2O), carbon dioxide (CO2), and methane (CH4). In these molecules, the atoms share electrons to achieve stability. For instance, in water, each hydrogen atom shares an electron with the oxygen atom, forming a stable molecule.

Why are covalent bonds important in biological molecules?

Covalent bonds are crucial in biological molecules because they provide the stability and specificity needed for complex structures and functions. For example, DNA, proteins, and carbohydrates all rely on covalent bonds to maintain their structures. These bonds ensure that biological molecules can perform essential functions such as replication, catalysis, and energy storage.

How can you determine the number of covalent bonds an atom can form?

The number of covalent bonds an atom can form is determined by its valency, which is the number of electrons needed to fill its outer shell. For example, carbon has four valence electrons and needs four more to complete its outer shell, allowing it to form four covalent bonds. Understanding an element's position in the periodic table can help predict its bonding behavior, which is a key concept in the Cambridge IGCSE Coordinated Science curriculum.

Why is "Saying covalent bonds are weak to explain low boiling points" a common mistake in Molecules and Covalent Bonds?

Why it happens: Students attribute the low melting/boiling point to weak covalent bonds rather than weak intermolecular forces. How to avoid it: Covalent bonds within the molecule are STRONG. The intermolecular forces BETWEEN molecules are weak. Always specify 'intermolecular forces' not 'covalent bonds'. Source: 0654 Examiner Report 2023

Why is "Saying diamond conducts electricity" a common mistake in Molecules and Covalent Bonds?

Why it happens: Students know diamond is a carbon allotrope like graphite, which does conduct. How to avoid it: Diamond: every electron is in a C–C bond, no free electrons, non-conductor. Graphite: one free/delocalised electron per carbon, conductor.

Why is "Drawing dot-and-cross diagrams with the wrong number of electrons in the shared bond" a common mistake in Molecules and Covalent Bonds?

Why it happens: Students put all electrons of one atom in the bond circle without including the contribution of the other atom. How to avoid it: A single bond = 2 electrons (1 from each atom). Double bond = 4 electrons (2 pairs). Count outer electrons carefully before drawing.

Atoms, Elements and CompoundsCambridge IGCSE Coordinated Sciences 0654

Molecules and Covalent Bonds

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.

Short Study Notes — Molecules and Covalent Bonds

Start with these resources to cover the key concepts, then work through the practice questions.

Page 1 / 0

Detailed 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 Molecules and Covalent Bonds, ready to print or save offline.

Covalent Bonding — Cambridge IGCSE Coordinated Science 0654

Covalent bonds form when non-metal atoms share pairs of electrons. Cambridge tests dot-and-cross diagrams, simple molecular vs giant covalent structures, and how structure explains physical properties.

What you’ll learn

Mapped to the Cambridge IGCSE 0654 syllabus (2025-2027).

C3.12 — Describe covalent bonding in terms of electron sharing.
C3.13 — Draw dot-and-cross diagrams for simple covalent molecules.
C3.14 — Explain properties of simple molecular and giant covalent substances.
C3.15 — Compare diamond and graphite structures.

Covalent Bonding — Electron Sharing

Covalent bonds form between non-metal atoms. Each bond is a shared pair of electrons, giving each atom a full outer shell.

Covalent bond: the electrostatic attraction between a shared pair of electrons and the nuclei of the bonded atoms. Forms between non-metal atoms.

Key molecules and their dot-and-cross diagrams:

Molecule	Formula	Bonds	Structure
Hydrogen	H₂	1 single bond	H–H
Water	H₂O	2 single bonds	V-shaped
Ammonia	NH₃	3 single bonds	Pyramidal
Methane	CH₄	4 single bonds	Tetrahedral
Chlorine	Cl₂	1 single bond	Cl–Cl
Oxygen	O₂	1 double bond	O=O
Carbon dioxide	CO₂	2 double bonds	O=C=O (linear)
Hydrogen chloride	HCl	1 single bond	H–Cl

Drawing dot-and-cross diagrams:

Use dots for one atom's electrons, crosses for the other's
Show only outer shell electrons
Shared pair appears between the two atoms
All atoms should achieve noble gas configuration (usually 8 outer electrons, except H which needs 2)

Covalent bond: shared electron pair between non-metal atoms.
Single bond: 1 shared pair. Double bond: 2 shared pairs.
Both atoms achieve full outer shell (noble gas configuration).

Simple Molecular Structures

Simple molecules have strong covalent bonds within the molecule but weak intermolecular forces between molecules — hence low melting/boiling points.

Simple molecular substances: H₂O, CO₂, NH₃, CH₄, HCl, O₂, Cl₂, N₂, iodine (I₂).

Properties:

Property	Explanation
Low melting and boiling points	Weak intermolecular forces between molecules — easy to overcome
Often gases or liquids at room temperature	Weak IMF
Do NOT conduct electricity	No charged particles free to move (molecules are neutral)
Often soluble in organic solvents	Similar intermolecular forces (like dissolves like)
Some dissolve in water	If molecules can form hydrogen bonds or ionise (e.g. HCl → H⁺ + Cl⁻)

IMPORTANT distinction: The COVALENT BONDS within the molecule are strong (not broken at MP or BP). The INTERMOLECULAR FORCES between molecules are weak and are broken at MP/BP.

Simple molecular: low MP/BP (weak intermolecular forces). Do not conduct electricity.
Strong covalent bonds within molecule are NOT broken at MP/BP — only intermolecular forces are overcome.

Giant Covalent Structures

Diamond, graphite, and silicon dioxide have millions of covalent bonds throughout — explaining their extremely high melting points.

Giant covalent (macromolecular) structures contain millions of covalently bonded atoms in a 3D network.

Diamond:

Each C bonded to 4 other C atoms → tetrahedral
All 4 outer electrons used in bonding → no free electrons → does NOT conduct electricity
Very hard (all bonds must be broken to deform) — used in cutting tools
Very high MP (~3550°C) — many strong covalent bonds throughout

Graphite:

Each C bonded to 3 other C atoms → forms layers of hexagonal rings
4th outer electron is delocalised (free to move) → conducts electricity (used in electrodes)
Layers held together by weak van der Waals forces → layers slide easily → lubricant, pencil 'lead'
High MP (strong bonds within layers) but soft (layers slide)

Silicon dioxide (SiO₂):

Each Si bonded to 4 O atoms; each O bonded to 2 Si atoms
Giant structure — very high MP (1610°C)
Hard, does not conduct (no free electrons)
Very hard to dissolve (many bonds to break)

Diamond: 4 bonds per C, very hard, does NOT conduct (no free electrons).
Graphite: 3 bonds per C, delocalised electrons → conducts, layers slide → lubricant.
SiO₂: giant structure, high MP, hard, does not conduct.

How it’s examined

Paper 4: Draw dot-and-cross diagram for H₂O or CO₂ (2 marks). 'Explain why diamond has a high melting point' (2 marks — giant covalent, many strong covalent bonds). 'Explain why graphite conducts electricity but diamond does not' (3 marks — delocalised electrons in graphite, free to move; diamond has no free electrons). 'Explain why CO₂ is a gas at room temperature' (2 marks — simple molecular, weak intermolecular forces). These are among the most commonly tested chemistry topics.

Sources: Cambridge IGCSE Coordinated Sciences 0654 syllabus 2025-2027 (C3); 0654 Examiner Reports 2022-2024. Last reviewed 2026-05-14.

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 Molecules and Covalent Bonds, ready to print or save as PDF.

Step-by-step worked examples — Molecules and Covalent Bonds

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

1Dot-and-cross diagram of water
Core
Question
Draw the dot-and-cross diagram of a water molecule (H₂O), showing all outer-shell electrons.
Step-by-step solution
1. Step 1
  Oxygen has 6 outer electrons; each hydrogen has 1 outer electron.
2. Step 2
  Oxygen shares one electron with each hydrogen atom, forming two single O–H covalent bonds.
3. Step 3
  After sharing, oxygen has 4 bonding electrons (2 pairs) and 4 non-bonding electrons (2 lone pairs); each hydrogen has 2 electrons (full first shell).
Answer
H₂O: oxygen at centre with two O–H single bonds and two lone pairs; each H has a shared pair giving full first shell.
2Why methane has a low boiling point
Extended
Question
Explain why methane (CH₄) has a low boiling point of $-161\,°\text{C}$ .
Step-by-step solution
1. Step 1
  Methane is a simple molecular substance: small CH₄ molecules with strong covalent bonds within each molecule.
2. Step 2
  Between molecules, only weak van der Waals (London dispersion) forces act.
3. Step 3
  Very little energy is needed to overcome these weak intermolecular forces, so methane has a very low boiling point.
Answer
Methane is a simple molecular substance with only weak intermolecular (van der Waals) forces between molecules; little energy is needed to overcome them, giving a low boiling point.
Examiner tip
Do NOT say 'weak covalent bonds' — the covalent bonds within the molecule are STRONG. It is the forces BETWEEN molecules that are weak.
3Diamond vs graphite electrical conductivity
Extended
Question
Compare the structures of diamond and graphite and explain the differences in their electrical conductivity.
Step-by-step solution
1. Step 1
  Diamond: each carbon atom is covalently bonded to four others in a rigid, three-dimensional tetrahedral lattice. All outer electrons are involved in C–C bonds; there are no free/delocalised electrons.
2. Step 2
  Because diamond has no free electrons, it is a non-conductor of electricity.
3. Step 3
  Graphite: each carbon is bonded to three others in flat hexagonal layers. Each carbon atom has one unhybridised electron that is delocalised across the layers.
4. Step 4
  These delocalised electrons are free to move through the layers, allowing graphite to conduct electricity.
Answer
Diamond: all electrons in C–C bonds, no free electrons → non-conductor. Graphite: one delocalised electron per C, free to move between layers → conductor.
Examiner tip
Use the words 'delocalised' and 'free to move'. These are mark-scheme key words.
4Triple bond in nitrogen
Core
Question
Nitrogen exists as N₂ molecules. Explain what is meant by a triple bond in N₂ and state why N₂ is very unreactive.
Step-by-step solution
1. Step 1
  A triple bond consists of three shared pairs of electrons (6 electrons shared) between the two nitrogen atoms.
2. Step 2
  The triple bond is very strong (bond energy ~944 kJ/mol), requiring a very large amount of energy to break.
3. Step 3
  Because so much energy is needed to break the N≡N bond, nitrogen is very unreactive at room temperature.
Answer
Triple bond = three shared pairs of electrons; very strong bond; requires large energy input to break → N₂ is very unreactive.

Key Definitions and Keywords — Molecules and Covalent Bonds

Definitions to memorise and the exact keywords mark schemes credit for molecules and covalent bonds answers — sharpened from recent examiner reports for the 2026 0654 sitting.

Covalent bond
Examiner keyword
A bond formed by the sharing of a pair of electrons between two non-metal atoms; each atom contributes one electron to the shared pair.
Simple molecular structure
Examiner keyword
A structure in which molecules consist of a small, fixed number of atoms held together by strong covalent bonds; the molecules themselves are attracted by weak intermolecular forces, giving low melting/boiling points.
Giant covalent structure
Examiner keyword
A structure in which a very large number of atoms are all linked by covalent bonds in a continuous three-dimensional (or layered) network; examples include diamond, graphite, and silicon dioxide.
Lone pair
A pair of electrons in the outer shell of an atom that is not involved in a covalent bond.
Delocalised electrons
Examiner keyword
Electrons that are not associated with a single atom or bond but are spread (delocalised) over a larger region; responsible for electrical conductivity in graphite and metals.
Van der Waals forces
Weak intermolecular attractions between all molecules, arising from temporary fluctuations in electron distribution; they increase with molecular size.

Common Mistakes and Misconceptions — Molecules and Covalent Bonds

The traps other students keep falling into on molecules and covalent bonds questions — taken from recent Cambridge IGCSE 0654 examiner reports and mark schemes — and how to avoid them.

✕Saying covalent bonds are weak to explain low boiling points
0654 Examiner Report 2023
Why it happens
Students attribute the low melting/boiling point to weak covalent bonds rather than weak intermolecular forces.
How to avoid it
Covalent bonds within the molecule are STRONG. The intermolecular forces BETWEEN molecules are weak. Always specify 'intermolecular forces' not 'covalent bonds'.
✕Saying diamond conducts electricity
Why it happens
Students know diamond is a carbon allotrope like graphite, which does conduct.
How to avoid it
Diamond: every electron is in a C–C bond, no free electrons, non-conductor. Graphite: one free/delocalised electron per carbon, conductor.
✕Drawing dot-and-cross diagrams with the wrong number of electrons in the shared bond
Why it happens
Students put all electrons of one atom in the bond circle without including the contribution of the other atom.
How to avoid it
A single bond = 2 electrons (1 from each atom). Double bond = 4 electrons (2 pairs). Count outer electrons carefully before drawing.

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.