rosie matthews

low melting points and boiling points.

• Most substances with simple molecules have low melting pointsand boiling points. Many are in the liquid or gas state at room temperature.
• Melting and boiling
• Melting and boiling are changes of state.
• Energy must be transferred to a substance to make it melt 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 forces between small molecules. Intermolecular forces are much weaker than the strong covalent bonds 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.

low melting point and boiling points

• 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 particles, 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.

Modelling molecules

• Structural formulae
• A structural formula shows the bonds between the atoms in a molecule. In these formulae:
• each atom is shown by its chemical symbol
• each covalent bond is shown as a straight line
• For example, an ammonia molecule has three covalent bonds. The diagram shows its structure.
• Three-dimensional structures
• The structure of a small molecule can also be shown as a three-dimensional ball and stick model. These models show how the atoms and bonds are arranged in space. The diagram shows a ball and stick model of ammonia.

Intermolecular forces

• Simple covalent compound
• 1.Is made of 2 or more non metals bonded together
• 2.Always a gas or liquid at room temp
• 3.Only contains a few atoms
• Definition:
• Weak forces of attraction between the molecules
• So only a small amount of energy is needed to separate the molecules
• So this simple molecule can be a gas at room temperature