Simple molecular substances
The physical propertiesA description of the appearance of a substance or how it acts without involving chemical reactions. For example, state, melting point, conductivity, etc. of simple molecular substances can be explained by thinking about their structure and bonding.
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 . 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. within the 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. They are often liquids or gases at room temperature.
Differences in melting and boiling points
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 intermolecular forces of attraction between the molecules.
- some intermolecular forces 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 intermolecular forces 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 pointThe temperature at which a substance rapidly changes from a liquid to a gas..
For example, the longer the alkane molecule, the higher the boiling point.
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
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.