Properties of substances with small molecules
A substance with small molecules has strong covalent bondA bond between atoms formed when atoms share electrons to achieve a full outer shell of electrons. that hold the atoms together in its moleculeA collection of two or more atoms held together by chemical bonds.. There are weak forces between molecules and their neighbours. The structure and bonding of substances with small molecules explains their propertiesThe characteristics of something. In chemistry, chemical properties include the reactions a substance can take part in. Physical properties include colour and boiling point..
Low melting points and boiling points
Most substances with simple molecules have low melting pointThe temperature at which a solid changes into a liquid as it is heated. and boiling pointThe temperature at which a substance rapidly changes from a liquid to a gas.. Many are in the liquid or gas state at room temperature.
Melting and boiling
Melting and boiling are changes of state.
energyThe capacity of a system to do work or the quantity required for mechanical work to take place. Measured in joules (J). For example, a man transfers 100 J of energy when moving a wheelbarrow. must be transferred to a substance to make it meltingThe process that occurs when a solid turns into a liquid when it is heated. or boil. This energy overcomes the attractive forces between the particles in the substance:
- some forces of attraction are overcome during melting, allowing molecules to move over each other
- more of the forces of attraction are overcome during boiling, allowing the molecules to move freely away from each other
The more energy that is needed, the higher the melting point or boiling point.
Explanation
There are intermolecular forcesWeak attractive forces between molecules. When a simple molecular substance melts or boils, it is the intermolecular forces that are broken (not the covalent bonds in each molecule). between small molecules. Intermolecular forces are much weaker than the strong covalent bondA bond between atoms formed when atoms share electrons to achieve a full outer shell of electrons. in molecules. When small molecular substances melt or boil, it is these weak intermolecular forces that are overcome. The covalent bonds are not broken. Relatively little energy is needed to overcome the intermolecular forces, so small molecular substances have low melting and boiling points.
| Substance | Melting point | Boiling point | State at 20掳颁 |
| O2 | -218掳颁 | -183掳颁 | Gas |
| H2O | 0掳颁 | 100掳颁 | Liquid |
| Substance | O2 |
|---|---|
| Melting point | -218掳颁 |
| Boiling point | -183掳颁 |
| State at 20掳颁 | Gas |
| Substance | H2O |
|---|---|
| Melting point | 0掳颁 |
| Boiling point | 100掳颁 |
| State at 20掳颁 | Liquid |
The intermolecular forces between water molecules are stronger than those between oxygen molecules.
In general, the bigger the molecule, the stronger the intermolecular forces, so the higher the melting and boiling points.
Conduction of electricity
A substance can conduct electricity if:
- it contains charged particlesParticles, usually ions or electrons, that carry electrical charges., and
- these particles are free to move from place to place
Small molecules have no overall electric charge, so they cannot conduct electricity, even when liquid or dissolved in water.