The Relationship of Bonding and Structure with the Properties of Substances

1. Which of the following compounds has the strongest forces of attraction between its particles?

• Methane (CH4 ) boiling point = –162°C
• Silica (SiO2 ) boiling point = 2230°C
• Sodium chloride (NaCl) boiling point = 1465°C

Answer: Silica

2. What type of particle is responsible for carrying the charge when molten sodium chloride conducts electricity?

Answer: ions

3. What particles are responsible for carrying the charge when solid aluminium conducts electricity?

Answer: delocalised electrons

4. Why are most covalent substances unable to conduct electricity?

Answer: They do not contain charged particles that are free to move.

5. Give one property of metallic substances that is not shared by ionic substances.

Answer: are malleable or conduct electricity when solid

6. The table below shows the melting and boiling points of three elements.

a) Give the state of matter of argon at –190°C.

Answer: Solid

b) Which element is a liquid over the widest range of temperature?

Answer: Oxygen

c) A mixture of these elements was cooled to –220°C and then warmed slowly. Predict which element would boil first. Explain your answer.

Answer: Nitrogen because it has the lowest boiling point.

d) A student thinks that nitrogen and oxygen consist of very small molecules. Give evidence from the table above to support this idea.

Answer: Nitrogen and oxygen have low melting and boiling points. Small molecules only have weak intermolecular forces between them that do not need a lot of energy to overcome, so they have low melting and boiling points.

7. Carbon dioxide, CO2, is a gas at room temperature while silicon dioxide (silica), SiO2 , is a solid at room temperature. Silica has a very high melting point.

a) Give the type of bonding that occurs between atoms of carbon and oxygen. Explain your answer.

Answer: Covalent bonding  because carbon and oxygen are both nonmetal elements.

b) Both carbon dioxide and silica have poor electrical conductivity. Suggest a reason for this.

Answer: The particles/molecules/atoms are neutrally charged/they do not contain ions/delocalised electrons which are free to move

c) Explain in terms of structure and bonding why carbon dioxide and silica are different states at room temperature.

Answer: Carbon dioxide is made of small molecules. Between these molecules are only weak intermolecular forces of attraction. Very little energy is needed to overcome these forces, so at room
temperature carbon dioxide is a gas. Silica has a giant covalent structure. Between the atoms are strong covalent bonds. A lot of heat energy is required to overcome these covalent bonds, so silica has a high melting point.

8. Many metals and alloys are used for a wide variety of purposes. The bonding and structure of metals and alloys make them very useful materials. The figure below is a representation of the bonding in a pure metal.

a) Explain why metals are good electrical and thermal conductors. Give your answer in terms of structure and bonding.

Answer: Metals have delocalised electrons in their structure. Electricity is the flow of charged particles, and the delocalised electrons are free to move, so metals conduct electricity. The delocalised electrons also transfer thermal energy so metals conduct heat.

b) Explain in terms of structure and bonding why metals are malleable while alloys tend to be harder than pure metals. Include a diagram in your answer.

Answer: Metals are malleable because their atoms are arranged in layers. These layers are able to slide over each other easily so metals can be bent or hammered into shape. Alloys are metals with other elements added to them. These different atoms disrupt the lattice structure preventing the layers from sliding over each other. This makes the alloy harder. Either of the following two diagrams could be used to support the answer.

9. The table below shows the properties of three different types of compound.

a) Suggest a reason why all three substances have high melting and boiling points.

Answer: They have strong forces of attraction between their particles.

b) Explain the difference in the electrical properties of these three compounds. Give your answer in terms of structure and bonding.

Answer: Substance A is a metal. Metals contain delocalised electrons that are free to move and carry the charge when solid and liquid, so metals conduct when solid and liquid. Substance B is ionic. The ions can only flow when the substance is a liquid so it doesn’t conduct when solid. Substance C is giant covalent. There are no charged particles that can flow, so it doesn’t conduct electricity.

10. Potassium bromide, KBr, can be made directly by reacting potassium with bromine. Potassium is in Group 1 of the periodic table. Bromine is in Group 7 of the periodic table. The figure below shows a representation of how the particles are arranged in solid potassium bromide.

a) Describe the type of bonding and structure represented in this figure.

Answer: Ionic bonding. Oppositely charged ions are held in a giant lattice structure by strong forces of attraction between them.

b) Potassium bromide does not conduct electricity when solid, but does conduct electricity when molten or dissolved in water. Explain the electrical properties of potassium bromide. Answer in terms of structure and bonding.

Answer: Electricity is the flow of charged particles. In potassium bromide the ions carry the charge. When solid the ions are not free to move so charge cannot flow. When molten or dissolved the ions are free to move, so charge can flow.

c) The table below shows the melting points of potassium bromide and some of the other Group 1 metal bromides. A student says, "Larger ions form weaker forces of attraction than smaller ions". Do you agree? Use this data and your knowledge of Group 1 elements to justify your answer.

Answer: Going down the Group 1 bromides the melting point decreases. The force of attraction between the ions must therefore be getting weaker down Group 1. Going down Group 1 we add an extra shell of electrons to each ion, so they get bigger. This matches the statement that bigger ions have weaker forces of attraction.