SIMPLE MOLECULAR SUBSTANCES are made of small, discrete molecules — each molecule contains a fixed number of atoms joined by STRONG COVALENT BONDS.
Examples:
Water (H₂O) — 3 atoms per molecule.
Carbon dioxide (CO₂) — 3 atoms per molecule.
Methane (CH₄) — 5 atoms per molecule.
Hydrogen (H₂) — 2 atoms per molecule.
Ammonia (NH₃) — 4 atoms per molecule.
Oxygen (O₂), nitrogen (N₂), chlorine (Cl₂) — all diatomic.
The COVALENT BONDS within each molecule are STRONG — but this is NOT what determines melting/boiling point.
Why Simple Molecular Substances Have Low Melting Points
The KEY to understanding simple molecular properties:
There are TWO types of forces to consider:
1. COVALENT BONDS — within each molecule, between atoms. These are STRONG.
2. INTERMOLECULAR FORCES — between separate molecules. These are WEAK.
When a simple molecular substance melts or boils:
The INTERMOLECULAR FORCES between molecules are overcome — molecules separate from each other.
The COVALENT BONDS within molecules are NOT broken.
Because intermolecular forces are WEAK, only a small amount of energy is needed → LOW melting and boiling points.
Examples:
H₂O boils at 100°C.
CH₄ boils at −161°C.
O₂ boils at −183°C.
Most simple molecular substances are GASES or LIQUIDS at room temperature.
Larger molecules have more electrons → stronger intermolecular forces → higher melting/boiling points.
Other Properties of Simple Molecular Substances
DO NOT CONDUCT ELECTRICITY:
Molecules have no overall charge.
There are no free electrons or ions to carry charge.
This applies whether solid, liquid or dissolved — unless the substance is ionic.
SOLUBILITY:
'Like dissolves like' — non-polar molecules tend to dissolve in non-polar solvents; polar molecules dissolve in polar solvents (like water).
Some simple molecules dissolve in water (e.g. sugar, HCl gas, CO₂).
Others are insoluble (e.g. iodine in water, but iodine dissolves well in hexane).
Note: when HCl gas dissolves in water, it IONISES:
HCl(g) + water → H⁺(aq) + Cl⁻(aq)
The resulting solution (hydrochloric acid) DOES conduct electricity — because ions are present.
This shows that dissolving can sometimes create ions, even from covalent molecules.
⚠️ Common Mistake
Simple molecular substances have LOW melting points because INTERMOLECULAR FORCES between molecules are weak — NOT because covalent bonds are weak. The covalent bonds WITHIN molecules are actually very strong. The confusion between these two types of force is one of the most common errors in this topic.
📌 Key Note
Simple molecules: small, discrete, strong covalent bonds within. Low MP/BP because WEAK intermolecular forces between molecules. Larger molecules → stronger intermolecular forces → higher MP/BP. No electrical conduction (no free electrons or ions). Melting/boiling separates molecules — does NOT break covalent bonds.
🎯 Matching Activity — Simple Molecular vs Giant Structure
Sort each property into simple molecular substance or giant structure. — drag the symbols on the right to match the component names on the left.
Simple molecular
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Giant structure
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Simple molecular
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Simple molecular
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Giant structure (ionic)
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Very high melting point — many strong bonds throughout the entire structure
Low melting point — weak intermolecular forces between molecules overcome easily
Conducts when molten or dissolved — free ions carry charge
Often gases or liquids at room temperature
Does not conduct electricity — no free ions or electrons
🎯 Test Yourself
Question 1 of 2
1. Water (H₂O) boils at 100°C, but methane (CH₄) boils at −161°C. Both are simple molecular substances. Why is the difference so large?
2. Why do simple molecular substances not conduct electricity?
⭐ How Well Do You Understand This Topic?
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