Properties of simple molecular substances
The physical propertiesThe characteristics of something. In chemistry, chemical properties include the reactions a substance can take part in. Physical properties include colour and boiling point. of simple molecularRefers to a substance made up of molecules, so it contains small groups of atoms, of a set size, held together by covalent bonds. substances can be explained by thinking about their structure and bonding.
Low melting points and boiling points
Simple molecular substances generally 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. and are often liquids or gases at room temperature.
Melting and boiling
Melting and boiling are state changes.
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. is transferred to a substance to meltingThe process that occurs when a solid turns into a liquid when it is heated. or boil it. This energy is needed to overcome the bonds or forces of attraction between the particleA general term for a small piece of matter. For example, protons, neutrons, electrons, atoms, ions or molecules. in the substance:
- some forces of attraction are overcome during melting, allowing moleculeA collection of two or more atoms held together by chemical bonds. 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 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 simple molecules. These 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 simple molecular substances melt or boil, it is these weak intermolecular forces that are overcome. The covalent bonds are not broken. Very little energy is needed to overcome the intermolecular forces, so simple molecular substances usually 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.
Conduction of electricity
A substance can conductTo allow electricity, heat or other energy forms to pass through. 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
Simple molecules have no overall chargeProperty of matter that causes a force when near another charge. Charge comes in two forms, positive and negative. For example, a negative charge causes a repulsive force on a neighbouring negative charge., or charged particles that can separate, so simple molecular substances cannot conduct electricity, even when liquid or dissolved in water.
More guides on this topic
- Equations and formulae - Edexcel
- Hazards and risks - Edexcel
- Atomic structure - Edexcel
- The periodic table - Edexcel
- Ionic compounds - Edexcel
- Giant covalent substances - Edexcel
- Metals and non-metals - Edexcel
- Chemistry calculations - Edexcel
- Mole calculations (higher) - Edexcel
- Sample exam questions - key concepts in chemistry - Edexcel