What Are Simple Molecular Substances?

Simple molecular substances are substances made up of small molecules held together by covalent bonds.

In these substances:

• Atoms are joined by strong covalent bonds within each molecule
• Molecules are held together by weak intermolecular forces

Because the forces between molecules are weak, simple molecular substances often have low melting and boiling points.

Examples include:

• Oxygen (O₂)
• Carbon dioxide (CO₂)
• Water (H₂O)
• Methane (CH₄)

Structure of Simple Molecular Substances

Simple molecular substances consist of discrete molecules.

Within each molecule:

• Atoms are connected by covalent bonds
• Electrons are shared between atoms

However, the attraction between separate molecules is much weaker than the covalent bonds within molecules.

These weak forces are known as intermolecular forces.

Intermolecular Forces

Intermolecular forces are the forces of attraction between molecules.

These forces are much weaker than covalent bonds, which explains many properties of molecular substances.

Because these forces are weak:

• Less energy is needed to separate molecules
• Substances melt or boil at relatively low temperatures

Properties of Simple Molecular Substances

Simple molecular substances have several common properties.

Low Melting and Boiling Points

These substances usually have low melting and boiling points.

This is because only weak intermolecular forces need to be overcome when the substance melts or boils.

Example:

Carbon dioxide exists as a gas at room temperature.

Poor Conductors of Electricity

Simple molecular substances usually do not conduct electricity.

This is because they do not contain free ions or electrons that can carry electric charge.

Example:

Pure water is a poor conductor of electricity.

Often Gases or Liquids

Because of weak intermolecular forces, many molecular substances are:

• Gases (e.g., oxygen, nitrogen)
• Liquids (e.g., water)

Some can also be soft solids.

Examples of Simple Molecular Substances

Oxygen (O₂)

Oxygen consists of two oxygen atoms joined by a double covalent bond.

It is a gas at room temperature.

Carbon Dioxide (CO₂)

Carbon dioxide contains double covalent bonds between carbon and oxygen atoms.

It is commonly found in air and used in carbonated drinks.

Methane (CH₄)

Methane is a simple molecular compound made of carbon and hydrogen atoms.

It is an important fuel used in natural gas.

Comparison with Giant Covalent Structures

Simple molecular substances differ from giant covalent structures.

FeatureSimple MolecularGiant CovalentStructureSmall moleculesGiant latticeBondingCovalent bonds in moleculesCovalent bonds throughout structureMelting pointLowVery highElectrical conductivityPoorUsually poor

Example:

Diamond is a giant covalent structure, while water is a simple molecular substance.

Importance of Molecular Substances

Simple molecular substances are important in many areas of chemistry and everyday life.

Examples include:

• Water in biological systems
• Oxygen used in respiration
• Carbon dioxide in photosynthesis
• Methane as a fuel

Understanding these substances helps explain many natural and chemical processes.

Exam Tip (5070)

Students are often asked to:

• Define simple molecular substances
• Explain their low melting and boiling points
• Compare them with giant structures

Example exam question:

Why do simple molecular substances have low boiling points?

Answer:

Because only weak intermolecular forces need to be overcome when molecules separate.

Practice Question

Why do simple molecular substances not conduct electricity?

Answer

They do not conduct electricity because they do not contain free ions or delocalised electrons.

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