Who this is for: Cambridge IGCSE Coordinated Science (0654) students who want molecules and covalent bonds — how non-metals share electrons — to become a reliable source of marks instead of a dot-and-cross memorisation exercise.
What query it owns: how to understand and revise molecules and covalent bonds in Cambridge IGCSE Coordinated Science.
Why this is safe: this page owns the molecules-and-covalent-bonds revision-guide angle, while Tutopiya’s Molecules And Covalent Bonds subtopic page owns the learning resource and the free Molecules And Covalent Bonds quiz owns the practice.

Covalent bonding joins non-metal atoms by sharing electrons. Cambridge IGCSE Coordinated Science (0654) expects you to draw dot-and-cross diagrams for molecules such as H₂, Cl₂, H₂O and CH₄, explain single and double bonds, and link simple molecular structures to low melting points and poor conductivity. This guide links each idea to what examiners reward.

Key takeaways

• A covalent bond forms when two non-metal atoms share one or more pairs of electrons.
• A single bond = one shared pair; a double bond = two shared pairs; a triple bond = three shared pairs.
• Simple molecular substances (e.g. CO₂, H₂O) have low melting/boiling points and do not conduct electricity.
• Each atom in a covalent bond aims for a full outer shell (usually 8 electrons — octet rule).
• Dot-and-cross diagrams show shared pairs between atoms and lone pairs where applicable.

What are molecules and covalent bonds in Cambridge IGCSE Coordinated Science?

When non-metal atoms bond together, neither atom has enough electrons to lose or gain to form ions. Instead, they share electrons so both achieve stable outer shells. The shared electrons form a covalent bond. Groups of atoms bonded covalently make molecules — discrete units with low intermolecular forces between them.

You can read the full explanation, dot-and-cross diagrams and notes on Tutopiya’s Molecules And Covalent Bonds subtopic page before you attempt questions.

Common covalent molecules — bonds and diagrams

Molecule	Atoms	Bonds	Shared pairs
H₂	2 H	Single	1 pair
Cl₂	2 Cl	Single	1 pair
H₂O	2 H + 1 O	2 single O–H	2 pairs (O also has 2 lone pairs)
CH₄	1 C + 4 H	4 single C–H	4 pairs
CO₂	1 C + 2 O	2 double O=C	4 pairs total (2 per double bond)
N₂	2 N	Triple	3 pairs

Properties of simple molecular substances

Property	Explanation
Low melting/boiling point	Weak intermolecular forces between molecules; little energy needed to separate them
Does not conduct electricity	No free-moving ions or electrons
Usually insoluble in water	Many are non-polar (exceptions like sugar and ethanol)
Soft / easily vaporised	Weak forces between molecules, not within them

Covalent bonds in past-paper wording: command words that matter

Command word / phrase	What the question wants	Typical covalent bonding stem
Draw	Dot-and-cross diagram	”Draw a dot-and-cross diagram for a water molecule.”
Explain how	Describe bond formation	”Explain how a covalent bond forms in a chlorine molecule.”
State	Give a fact	”State why covalent compounds do not conduct electricity.”
Compare	Ionic vs covalent	”Compare the melting points of NaCl and CO₂.”

Worked exam-style stems (how to answer the wording)

1. “Explain how a covalent bond forms in a hydrogen molecule, H₂.” Each hydrogen atom has 1 electron; they share one pair of electrons so both have a full outer shell (2 electrons). Mark-scheme reward: sharing electrons and stable outer shell.
2. “State why simple covalent substances have low melting points.” Weak intermolecular forces between molecules; little energy needed to overcome them (covalent bonds within molecules are strong). Reward: intermolecular forces and low energy.
3. “Draw a dot-and-cross diagram for methane, CH₄.” Carbon in centre with 4 shared pairs with 4 hydrogen atoms; each H has 2 electrons in outer shell, C has 8. Reward: correct sharing and full outer shells.

Test yourself with the Molecules And Covalent Bonds quiz once you can draw diagrams and explain molecular properties.

How covalent bonding connects to the rest of Coordinated Science

Covalent bonding contrasts with Ions And Ionic Bonds and leads into Macromolecules — giant covalent structures with very different properties. It underpins organic chemistry and Biological Molecules. The Cambridge IGCSE Coordinated Science resource hub links every subtopic.

Common mistakes students make

• Confusing intermolecular forces (weak, between molecules) with covalent bonds (strong, within molecules).
• Drawing ionic diagrams showing electron transfer for covalent molecules.
• Forgetting lone pairs on oxygen and nitrogen in diagrams.
• Saying covalent substances have high melting points (simple molecular ones have low melting points).
• Drawing too few electrons — check each atom has a full outer shell.

When you need more support

If covalent bonding questions keep costing marks, work through the Molecules And Covalent Bonds quiz, then get focused help from a Cambridge IGCSE Coordinated Science tutor.

Frequently asked questions

Is covalent bonding hard in Coordinated Science? The sharing model is clear — marks are lost on dot-and-cross diagrams and confusing intermolecular forces with covalent bonds.

What is the difference between a single and double covalent bond? A single bond shares one pair of electrons; a double bond shares two pairs between the same two atoms.

Why do simple covalent substances not conduct electricity? They have no free-moving ions or delocalised electrons to carry charge.

How do I revise molecules and covalent bonds effectively? Draw dot-and-cross diagrams for H₂O, CH₄ and CO₂ from memory, explain low melting points, then take the Molecules And Covalent Bonds quiz.

Ready to master Cambridge IGCSE Coordinated Science molecules and covalent bonds?

Start with the Molecules And Covalent Bonds subtopic page, then book a free trial with a Cambridge IGCSE Coordinated Science specialist to turn covalent bonding into guaranteed marks.

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