Properties of Simple Molecular Substances

Definition and Examples

Simple molecular substances are made up of small molecules, each containing a fixed number of atoms joined by covalent bonds. These substances consist of discrete molecules rather than giant networks or lattices.

Examples include:

• Water (H₂O)
• Carbon dioxide (CO₂)
• Iodine (I₂)
• Oxygen (O₂)
• Hydrogen (H₂)

In these substances, atoms are held together within each molecule by strong covalent bonds. However, the forces between the molecules themselves are much weaker (see next section).

Intermolecular Forces

Between the molecules of simple molecular substances, there are weak intermolecular forces. These include:

• Van der Waals forces (also called London dispersion forces) — present in all molecules, caused by temporary dipoles.
• Dipole-dipole forces — occur between polar molecules where permanent dipoles attract each other.

These intermolecular forces are much weaker than the covalent bonds within molecules and far weaker than ionic or metallic bonds.

The strength of these forces affects physical properties such as melting and boiling points. The stronger the intermolecular forces, the higher the melting and boiling points.

For example, iodine (I₂) is a solid at room temperature because its molecules have stronger van der Waals forces due to its large size, whereas oxygen (O₂) is a gas because its molecules have weaker intermolecular forces.

Physical Properties

Simple molecular substances have characteristic physical properties because of their weak intermolecular forces:

• Low melting and boiling points: Only a small amount of energy is needed to overcome the weak intermolecular forces, so these substances melt and boil at low temperatures.
• State at room temperature: Most are gases or liquids at room temperature (e.g., oxygen, nitrogen, water), although some larger molecules like iodine are solids.
• Electrical conductivity: They do not conduct electricity in solid or liquid states because they have no free charged particles (ions or electrons) to carry charge.

For instance, water boils at 100°C, carbon dioxide sublimes at -78°C, and iodine melts at 114°C, all relatively low compared to ionic or metallic substances.

The weak intermolecular forces also explain why simple molecular substances are often soft and brittle when solid, as the molecules can be separated easily. This is because the weak intermolecular forces mean molecules can be easily pulled apart, making the solids soft and brittle.

Solubility and Electrical Conductivity

Simple molecular substances tend to be soluble in non-polar solvents such as hexane or cyclohexane because “like dissolves like” — non-polar molecules dissolve well in non-polar solvents due to similar intermolecular forces.

Polar simple molecular substances (e.g., water) can dissolve in polar solvents like water itself, but many are insoluble in water if they are non-polar (e.g., iodine). This is because polar molecules interact through dipole-dipole forces and hydrogen bonding with polar solvents, while non-polar molecules interact via van der Waals forces with non-polar solvents.

In terms of electrical conductivity:

• Simple molecular substances do not conduct electricity in solid or liquid states because their molecules are neutral and there are no free ions or electrons.
• This contrasts with ionic compounds, which conduct electricity when molten or dissolved because ions are free to move.

Learning Example: Explaining the Low Boiling Point of Carbon Dioxide

Carbon dioxide (CO₂) is a simple molecular substance made of small molecules held together by covalent bonds. The molecules are linear and non-polar, so only weak van der Waals forces act between them. The linear shape of CO₂ means the dipoles cancel out, making it non-polar.

Because these intermolecular forces are weak, little energy is needed to separate the molecules, so CO₂ has a low boiling point of -78°C and exists as a gas at room temperature.