Properties of Small molecules

Small molecules are made up of a fixed and limited number of atoms which are held tightly together by strong covalent bonds. These molecules are different from giant covalent molecules, which have a large and variable number of atoms.

Melting and Boiling Points

Small covalent molecules often have low melting and boiling points. So they are typically gases or liquids at room temperature.

The following table shows the melting points, boiling points, and state at room temperature for several small covalent molecules:

Small moleculeMelting point (°C)Boiling point (°C)State at room temperature
Hydrogen (H2)-259-253Gas
Oxygen (O2)-218-183Gas
Methane (CH4)-182-161Gas
Ammonia (NH3)-78-33Gas

These small covalent molecules are gases at room temperature because they have boiling points below room temperature. However, there is one exception, which is water.

Small moleculeMelting pointBoiling Point (°C)State at room temperature
Water (H2O)0°C100°CLiquid

Solubility of covalent compounds

Covalent compounds are typically insoluble in water, meaning they do not dissolve and form a separate layer. However, some covalent compounds can dissolve in water because they form strong intermolecular forces of attraction with the water molecules.

These attractions are stronger than the attractions between the water molecules themselves and between the covalent molecules.

Some examples of covalent compounds that are soluble in water are:

  • Carbon dioxide (CO2)
  • Ammonia (NH3)
  • Sucrose (C12H22O11), also known as table sugar
Sugar (C12H22O11) in a spoon being poured into a cup of water (H2O).

Intermolecular Forces

The atoms in each molecule are held together by strong covalent bonds. However, there are also weak forces of attraction between individual molecules called intermolecular forces.

An illustration of two water molecules, represented by circles labelled 'O' and 'H', connected by dotted lines indicating weak intermolecular forces.

  • Intermolecular forces are generally weaker than covalent bonds

As the temperature increases, the vibration of the molecules increases. When the vibrations become strong enough, they can break the intermolecular forces that hold the molecules together. This causes the substance to boil and turn into a gas.

It does not take a lot of energy to break these weak intermolecular forces, compared to the energy required to break the covalent bonds within the molecule. So remember, it is the weak intermolecular forces that break, not the strong covalent bonds.

The strength of intermolecular forces depends on the size of the molecules. As the size of the molecules increases, so do the intermolecular forces. This means that more energy is required to overcome the forces of attraction between the larger molecules. As a result, larger molecules typically have higher melting and boiling points.

This relationship between molecular size and intermolecular forces can be seen in the boiling points of the first five alkanes. You can see this in the table below.

AlkaneFormulaBoiling Point (°C)State at room temperature
MethaneCH4-161Gas
EthaneC2H6-89Gas
PropaneC3H8-42Gas
ButaneC4H10-1Gas
PentaneC5H1236Liquid

The boiling points of these substances increase as their molecular size increases. So, intermolecular forces play a role in determining the physical properties of substances.

Electrical Conductivity

Small covalent molecules do not conduct electricity because they lack free ions or charged particles to carry the electric charge. As a result, small covalent molecules are considered to be good insulators, meaning they do not easily allow the flow of electricity.