What are simple molecules in the context of Cambridge IGCSE Chemistry?

In Cambridge IGCSE Chemistry, simple molecules refer to small groups of atoms bonded together by covalent bonds. These molecules typically consist of a few atoms, such as H2O (water), CO2 (carbon dioxide), and O2 (oxygen). They are characterized by strong covalent bonds within the molecule but weak intermolecular forces between molecules, leading to low melting and boiling points.

How do covalent bonds form between atoms?

Covalent bonds form when two atoms share one or more pairs of electrons to achieve a full outer shell, resembling the electron configuration of noble gases. This sharing of electrons allows each atom to attain stability. For example, in a water molecule (H2O), each hydrogen atom shares an electron with the oxygen atom, resulting in a stable covalent bond.

What is the difference between covalent bonds and ionic bonds?

Covalent bonds involve the sharing of electron pairs between atoms, typically between non-metal elements, resulting in the formation of simple molecules. In contrast, ionic bonds occur when electrons are transferred from one atom to another, usually between metal and non-metal elements, creating ions that attract each other due to opposite charges. This difference in bonding leads to distinct properties in the resulting compounds.

Why do simple molecules have low melting and boiling points?

Simple molecules have low melting and boiling points because they are held together by weak intermolecular forces, such as van der Waals forces or hydrogen bonds. These forces are much weaker than the covalent bonds within the molecules, requiring less energy to overcome. As a result, simple molecular substances are often gases or liquids at room temperature.

Can you explain the concept of a double covalent bond with an example?

A double covalent bond occurs when two pairs of electrons are shared between two atoms. This type of bond is stronger and shorter than a single covalent bond. An example is the oxygen molecule (O2), where each oxygen atom shares two electrons with the other, forming a double bond. This sharing allows both oxygen atoms to achieve a stable electron configuration.

Why is "Saying low melting point is due to weak covalent bonds" a common mistake in Simple Molecules and Covalent Bonds?

Why it happens: Confusing covalent bonds with intermolecular forces. How to avoid it: Covalent bonds are STRONG. Low m.p. of simple molecules is due to weak FORCES BETWEEN MOLECULES, not the bonds inside them. Source: 0620/42 — recurring

Why is "Saying simple molecular compounds conduct when liquid" a common mistake in Simple Molecules and Covalent Bonds?

Why it happens: Generalising from ionic compounds. How to avoid it: Simple molecular: NO mobile ions or free electrons → no conduction in any state (with rare exceptions).

Why is "Drawing electrons being transferred in a covalent bond" a common mistake in Simple Molecules and Covalent Bonds?

Why it happens: Mixing ionic with covalent. How to avoid it: Covalent = SHARED pair. The pair sits between the two atoms.

Why is "Drawing 8 electrons around hydrogen" a common mistake in Simple Molecules and Covalent Bonds?

Why it happens: Generalising the octet rule. How to avoid it: Hydrogen is happy with TWO outer electrons (helium configuration), not 8.

Atoms, Elements and CompoundsCambridge IGCSE Chemistry (0620)

Simple Molecules and Covalent Bonds

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Simple Molecules and Covalent Bonds — Cambridge IGCSE 0620 Chemistry Extended (2026)

Atoms sharing electrons, with weak intermolecular forces between molecules. Why simple molecular substances have low melting points and don't conduct.

What you’ll learn

Mapped to the Cambridge IGCSE 0620 syllabus (2026-2028).

2.2 — Describe the formation of covalent bonds by sharing of electron pairs.
2.2 — Draw dot-and-cross diagrams for simple molecules.
2.2 — Describe the properties of simple molecular substances.

What is a covalent bond?

Two non-metal atoms share a pair of electrons. Each ends up with a noble-gas configuration.

Covalent bond. A SHARED pair of electrons between two non-metal atoms.

Why share? Each atom is closer to a full outer shell (a noble-gas configuration). The shared electrons COUNT for BOTH atoms.

Worked: H₂ (hydrogen molecule).

Each H atom has 1 electron (config: 1).
They share both → each H 'sees' 2 electrons → like helium (full 1st shell).
Forms a single covalent bond H–H.

Worked: H₂O (water).

O has 6 outer e⁻; needs 2 more for full shell.
Each H shares 1 e⁻ with O.
O ends up with 8 (full); each H ends up with 2 (full).

Multiple bonds.

Single (1 pair shared): H₂, HCl, H₂O.
Double (2 pairs): O=O, CO₂ (each O double bonds to C: O=C=O).
Triple (3 pairs): N≡N (very strong; why N₂ is so unreactive).

Diagram conventions. Dots and crosses to distinguish electrons from each atom. A bond = a pair (one dot + one cross) between two atoms.

Cambridge tip. Always count BOTH the dots and crosses around an atom; should equal noble-gas configuration (2, 8, 8) when the molecule is correctly drawn.

Covalent bond = shared pair of electrons.
Both atoms 'see' the shared electrons.
Single, double, triple bonds.
Goal: full outer shell for each atom.

Properties of simple molecular substances

Strong covalent bonds INSIDE the molecule; weak forces BETWEEN molecules → low m.p., poor conductivity.

Two types of force in a simple molecular substance.

Within molecules: covalent bonds (strong).
Between molecules: weak intermolecular forces.

When you melt or boil a molecular substance, you only break the WEAK intermolecular forces — the strong covalent bonds INSIDE molecules are NOT broken.

Result: low m.p. and b.p. Often these substances are gases (O₂, N₂, CO₂) or liquids (H₂O, ethanol) at room temperature.

No conductivity. No free electrons; no free ions. Insulators in solid, liquid, and dissolved states.

Solubility. Some are soluble in water (e.g. sugar, ethanol — polar molecules); others are insoluble (e.g. oils, hexane — non-polar).

Worked qualitative. Why is the b.p. of H₂O ( $100\degree\text{C}$ ) much higher than CH₄ ( $-161\degree\text{C}$ ), even though both are simple molecules? Water has stronger intermolecular forces (specifically, hydrogen bonds). CH₄ has only weak van der Waals forces. The covalent bonds are not what's being broken in either case.

Cambridge wording. When asked "why does CO₂ have a low boiling point?" — say "The covalent bonds within CO₂ molecules are strong, but the FORCES BETWEEN molecules are weak. Only the weak forces need to be overcome to boil." Don't say "the covalent bonds break" — they don't, when you just boil the substance.

Strong covalent bonds INSIDE molecules.
Weak forces BETWEEN molecules.
Boiling breaks intermolecular forces, NOT bonds.
Low m.p. / b.p. → often gas or liquid.
Doesn't conduct: no free charges.

How it’s examined

Covalent bonding is examined every Paper 2 (4-5 marks: dot-and-cross diagrams, identify bond type) and every Paper 4 (6-8 marks: explain m.p./b.p., draw molecules, multiple bonds). Examiner reports flag two errors: missing electrons in dot-and-cross diagrams (need to total to noble gas), and saying 'covalent bonds break' when a substance simply melts.

Sources: Cambridge IGCSE Chemistry 0620 syllabus 2026-2028 (2.2); 0620/22 May/Jun 2024 — Q5 (covalent dot-and-cross); 0620 Examiner Reports 2022-2024. Last reviewed 2026-05-06.

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

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

1Dot-and-cross of $\text{H}_{2}$
Core• dot and cross
Question
Show the bonding in $\text{H}_{2}$ using a dot-and-cross diagram description.
Step-by-step solution
1. Step 1
  Each H has 1 outer electron — needs 2 (helium config) for stability.
2. Step 2
  The two atoms SHARE one pair of electrons → single covalent bond.
Answer
Two overlapping circles each containing one dot and one cross — a single shared pair.
2Dot-and-cross of $\text{H}_{2}\text{O}$
Extended• Adapted from 0620/42 May/Jun 2024 Q5• water
Question
Describe the bonding in water.
Step-by-step solution
1. Step 1
  O has 6 outer electrons; each H has 1.
2. Step 2
  Each H shares one pair with O → two single bonds. O also has TWO LONE PAIRS.
3. Step 3
  All three atoms reach noble-gas configurations.
Answer
$\text{H}\!-\!\text{O}\!-\!\text{H}$ with two lone pairs on O.
3Why simple molecular compounds have low melting points
Extended• properties
Question
Explain why methane melts at $-182\degree\text{C}$ but sodium chloride at $801\degree\text{C}$ .
Step-by-step solution
1. Step 1
  $\text{CH}_{4}$ has STRONG covalent bonds INSIDE the molecule but only WEAK intermolecular forces between molecules.
2. Step 2
  Melting only breaks the WEAK intermolecular forces → low melting point.
3. Step 3
  $\text{NaCl}$ requires breaking the strong electrostatic forces throughout the giant ionic lattice → much more energy.
Answer
Simple molecular: weak intermolecular forces → low m.p. Ionic lattice: strong electrostatic forces → high m.p.
4Double bond in $\text{O}_{2}$
Extended• double bond
Question
How many electron pairs are shared between the two atoms in $\text{O}_{2}$ ?
Step-by-step solution
1. Step 1
  Each O has 6 outer electrons. Each needs 2 more for the octet.
2. Step 2
  Sharing TWO pairs (a double bond) gives both atoms 8 outer electrons.
Answer
Two pairs shared (a $\text{O}\!=\!\text{O}$ double bond).

Key Formulae — Simple Molecules and Covalent Bonds

The formulae you need to memorise for simple molecules and covalent bonds on the Cambridge IGCSE 0620 paper, with every variable defined in plain English and a note on when to use it.

Octet (duet for H) rule
$\text{Atoms aim for outer-shell configurations of 8 (or 2 for H)}$
When to use
Working out covalent structures.

Key Definitions and Keywords — Simple Molecules and Covalent Bonds

Definitions to memorise and the exact keywords mark schemes credit for simple molecules and covalent bonds answers — sharpened from recent examiner reports for the 2026 0620 sitting.

Covalent bond
Examiner keyword
A shared pair of electrons between two non-metal atoms.
Simple molecular substance
Examiner keyword
Substance made of small, discrete molecules held together by weak intermolecular forces.
Double bond
Examiner keyword
Two pairs of electrons shared between two atoms (e.g. in $\text{O}_{2}$ , $\text{CO}_{2}$ ).
Lone pair
A pair of outer electrons on an atom NOT involved in a bond.

Common Mistakes and Misconceptions — Simple Molecules and Covalent Bonds

The traps other students keep falling into on simple molecules and covalent bonds questions — taken from recent Cambridge IGCSE 0620 examiner reports and mark schemes — and how to avoid them.

✕Saying low melting point is due to weak covalent bonds
0620/42 — recurring
Why it happens
Confusing covalent bonds with intermolecular forces.
How to avoid it
Covalent bonds are STRONG. Low m.p. of simple molecules is due to weak FORCES BETWEEN MOLECULES, not the bonds inside them.
✕Saying simple molecular compounds conduct when liquid
Why it happens
Generalising from ionic compounds.
How to avoid it
Simple molecular: NO mobile ions or free electrons → no conduction in any state (with rare exceptions).
✕Drawing electrons being transferred in a covalent bond
Why it happens
Mixing ionic with covalent.
How to avoid it
Covalent = SHARED pair. The pair sits between the two atoms.
✕Drawing 8 electrons around hydrogen
Why it happens
Generalising the octet rule.
How to avoid it
Hydrogen is happy with TWO outer electrons (helium configuration), not 8.

Practice questions

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Simple Molecules and Covalent Bonds — frequently asked questions

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Simple Molecules and Covalent Bonds

Short Study Notes in the form of Slides

Detailed Study Notes

What you’ll learn

How it’s examined

1Dot-and-cross of H2\text{H}_{2}H2​

2Dot-and-cross of H2O\text{H}_{2}\text{O}H2​O

3Why simple molecular compounds have low melting points

4Double bond in O2\text{O}_{2}O2​

Octet (duet for H) rule

Covalent bond

Simple molecular substance

Double bond

Lone pair

✕Saying low melting point is due to weak covalent bonds

✕Saying simple molecular compounds conduct when liquid

✕Drawing electrons being transferred in a covalent bond

✕Drawing 8 electrons around hydrogen

Practice questions

Past paper style quiz

Assess your understanding

Video lesson

Simple Molecules and Covalent Bonds — frequently asked questions

1Dot-and-cross of $\text{H}_{2}$

2Dot-and-cross of $\text{H}_{2}\text{O}$

4Double bond in $\text{O}_{2}$