2.5Atoms, Elements and Compounds

Simple molecules and covalent bonds — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions

5 min read 9 parts Lesson 5 of 7 1 quiz

📚 Cambridge IGCSE Chemistry 0620 Hub / Topical Revision

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Lesson summary

Simple molecules and covalent bonds — IGCSE CHEMISTRY notes: Describe covalent bonding as a shared pair of electrons, draw dot-and-cross diagrams of simple molecules, and explain their properties.Covalent bond — a shared pair of electrons between two atoms.Each bond is one shared…

Why atoms share
Hydrogen, H2
Water as a model
Dot and cross
Double bond
Triple bond

Part 1 of 7Covalent bond

Covalent bond

Covalent bond a shared pair of electrons between two atoms.
Each bond is one shared pair, which is 2 electrons.
The shared pair is attracted to both nuclei, and this attraction holds the atoms together.

Who bonds

Who bonds covalent bonding happens between non-metal atoms, which share electrons instead of transferring them.

Why atoms share

Why atoms share each atom gains a full outer shell, a noble gas electronic configuration.

Molecule

Molecule two or more atoms held together by covalent bonds;
- substances may be small molecules or giant molecules.

Hydrogen, H₂

Hydrogen, H₂ each H contributes 1 electron to one shared pair, so both have 2 electrons, like helium.

Atom	H	Cl	O	N	C
Bonds usually formed	1	1	2	3	4

Part 2 of 7Drawing dot-and-cross diagrams

Write the electronic configuration of each atom and count the outer-shell electrons.

Work out how many electrons each atom needs to fill its outer shell (H 1, C 4, N 3, O 2, Cl 1).

Place the atom needing the most bonds in the centre.

Draw overlapping outer shells and put one shared pair (one dot and one cross) in the overlap for every bond.

Add the remaining outer electrons as lone pairs and check that each atom has 8 outer electrons (H has 2).

(Supplement) If atoms still lack electrons, share two or three pairs to make double or triple bonds.

Water as a model

O has 6 outer electrons and each H has 1;
- overlap each H shell with the O shell, add two lone pairs on O, then check O has 8 and each H has 2.

Dot and cross

Dot and cross electrons from one atom are dots and electrons from the other are crosses, so the diagram shows where each electron came from.

Part 3 of 7Common simple molecules

Molecule	Formula	Bonds	Lone pairs
Hydrogen	H₂	1 single	none
Chlorine	Cl₂	1 single	3 on each Cl
Water	H₂O	2 single (O-H)	2 on O
Methane	CH₄	4 single (C-H)	none (C has 8 outer electrons)
Ammonia	NH₃	3 single (N-H)	1 on N
Hydrogen chloride	HCl	1 single	3 on Cl
(S) Oxygen	O₂	1 double	2 on each O
(S) Nitrogen	N₂	1 triple	1 on each N
(S) Carbon dioxide	CO₂	2 double (O=C=O)	2 on each O
(S) Ethene	C₂H₄	1 C=C, 4 C-H	none
(S) Ethane	C₂H₆	1 C-C, 6 C-H	none
(S) Methanol	CH₃OH	3 C-H, 1 C-O, 1 O-H	2 on O

Part 4 of 7Double and triple bonds (Supplement)

Double bond

Double bond two shared pairs (4 electrons), as in O=O, C=O in CO₂ and C=C in ethene.

Triple bond

Triple bond three shared pairs (6 electrons), as in N≡N.
The triple bond makes nitrogen very unreactive.

When used

When used atoms share more than one pair when a single pair does not give a full outer shell.

Part 5 of 7Properties of simple molecular substances

Properties of simple molecular substances

Low melting and boiling points

Low melting and boiling points only the weak intermolecular forces are overcome, and the covalent bonds inside the molecules stay intact.
Many are gases or liquids at room temperature (CH₄, H₂O, Cl₂).

Trend with size (Supplement)

Trend with size (Supplement) larger molecules have more electrons and stronger intermolecular forces, so melting and boiling points increase.
- (for example down the alkanes)

No electrical conductivity

No electrical conductivity molecules are uncharged and there are no mobile ions or electrons.
Plastic, rubber and wood are simple molecular insulators, which is why plastic coats electrical wires.

Feature	Covalent bonds	Intermolecular forces
Where they act	Within the molecule	Between molecules
Strength	Strong	Weak, about one tenth of a covalent bond
Effect of melting or boiling	Do not break	Overcome
Effect of larger molecules	No change	Increase

Part 6 of 7Simple molecular and ionic compounds compared

Property	Simple molecular	Ionic
Melting and boiling point	Low	High
Conductivity	Do not conduct in any state	Conduct when molten or aqueous
Overcome on melting	Weak intermolecular forces	Strong ionic bonds

Dot-and-cross feature	Ionic diagram	Covalent diagram
Elements	Metal and non-metal	Non-metals only
Outer shells	Separate ions	Overlapping circles
Square brackets	Round each ion	Not used
Charge	Shown on each ion	No charge shown
Electrons	Transferred	Shared as pairs in the overlap

Part 7 of 7More from the specification

1Covalent Bond Formation by Electron Sharing

A covalent bond consists of a shared pair of valence electrons between two adjacent non-metal atoms,
allowing both atoms to achieve stable, full outer electron shells.

2Physical Properties of Simple Molecular Compounds

Simple covalent molecules (like H₂O, CH₄,
- and CO₂) possess strong internal covalent bonds but very weak intermolecular forces,
- resulting in low melting and boiling points.

3Electrical Non-Conductivity of Covalent Substances

Because simple molecular substances consist of neutral molecules with no delocalised electrons or mobile ions, they cannot conduct electricity in any state.

4Compound

Two or more elements chemically combined in a fixed ratio.

Key terms

Term	Meaning
Covalent bond	A shared pair of electrons between two atoms, giving noble gas configurations.
Molecule	Two or more atoms held together by covalent bonds.
Single, double, triple bond	One, two or three shared pairs of electrons between two atoms.
Lone pair	A pair of outer-shell electrons not used in bonding.
Intermolecular forces	Weak attractions between separate molecules, overcome on melting or boiling.
Simple molecular substance	Small separate molecules with strong covalent bonds inside and weak forces between them.
Dot-and-cross diagram	Overlapping outer shells showing shared electrons, with dots and crosses for the two atoms.

Avoid these mistakes

Saying the O-H bonds break when water boils — the weak intermolecular forces between molecules are overcome.

Saying simple molecules have weak covalent bonds — covalent bonds are strong; the intermolecular forces are weak.

Saying a covalent bond is when atoms share electrons — it is a shared pair of electrons between two atoms.

Saying covalent compounds conduct when dissolved — simple molecular substances have no mobile ions or electrons.

Drawing ionic-style brackets and charges on a covalent molecule, or writing CO₂ with single bonds — it has two C=O double bonds.

Last updated: 18 October 2026