Intermolecular Bonding

Intermolecular Bonding

1. A substance with small molecules has strong covalent bonds that hold the atoms together in its molecules. There are weak forces between molecules and their neighbours. The structure and bonding of substances with small molecules explains their properties.

2. Low melting points and boiling points Most substances with simple molecules have low melting points and boiling points. Many are in the liquid or gas state at room temperature.

3. 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.

4. 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. Substance Melting point Boiling point State at 20°C O2 -218°C -183°C Gas H2O 0°C 100°C 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.

5. 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.

6. Modelling molecules Dot and cross diagrams A dot and cross diagram can show the bonding in a small molecule: the outer shell of each atom is drawn as a circle circles overlap where there is a covalent bond electrons from one atom are drawn as dots, and electrons from another atom as crosses For example, a hydrogen molecule, H2, forms when two hydrogen atoms each share their outer electron.

   • A dot and cross diagram to show the bonding in hydrogen

7. An ammonia molecule, NH3, forms when one nitrogen atom shares its outer electrons with three hydrogen atoms. There are two types of dot and cross diagram - one without circles, and one with.

   • These dot and cross diagrams show the bonding in ammonia

8. 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.

9. 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.

10. Interpreting dot and cross diagrams Dot and cross diagrams show how electrons are shared in covalent bonds. The table shows dot and cross diagrams for an element, chlorine, and two compounds.

11. Double and triple bonds Some molecules contain a double bond, which consists of two shared pairs of electrons. For example, oxygen molecules consist of two oxygen atoms joined together. Oxygen atoms can form two covalent bonds, so to link the two oxygen atoms together, a double bond forms between them. A nitrogen molecule is made up of two nitrogen atoms joined together. Nitrogen atoms can form three covalent bonds, so a triple bond forms between them. The structure of nitrogen is N≡N, showing that it has three shared pairs of electrons.

12. Properties of substances with small molecules A substance with small molecules has strong covalent bonds that hold the atoms together in its molecules. There are weak forces between molecules and their neighbours. The structure and bonding of substances with small molecules explains their properties.

13. Low melting points and boiling points Most substances with simple molecules have low melting points and boiling points. Many are in the liquid or gas state at room temperature.

14. 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.

15. 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.

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