Bonding and structure

AS Level Chemistry · States of matter. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.

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The idea

The physical properties of a solid depend on its particles and on what holds them together. To melt a giant structure (ionic, metallic or giant molecular) you must break strong bonds throughout the lattice, so the melting point is high. To melt a simple molecular solid you only overcome the weak forces between molecules, so the melting point is low.

A substance conducts electricity only if it has charged particles that are free to move: delocalised electrons in metals and graphite, or mobile ions in a molten or dissolved ionic compound.

In the exam you are often given properties and asked to work back to the type of structure.

Rules to remember

Common mistake

Students say that molten sodium chloride conducts because of free electrons. In ionic compounds the moving charges are ions; delocalised electrons carry the current only in metals and graphite.

Worked example

Solid Z melts at 114 °C. It does not conduct electricity as a solid or as a liquid. It dissolves well in hexane but hardly at all in water. What type of structure does Z have?

  1. A low melting point means only weak forces are overcome on melting, so Z is not a giant structure.
  2. No conduction in any state means there are no mobile ions and no delocalised electrons, so Z is not ionic or metallic.
  3. Dissolving in a non-polar solvent such as hexane fits non-polar molecules held together by id-id forces.

Answer: Simple molecular (for example, iodine).

Practice questions

Try each one, then open the answer.

1. Substance Q melts at 1700 °C, is insoluble in water and does not conduct electricity either as a solid or when molten. What type of structure does Q have?

  1. A
    giant ionic
  2. B
    giant metallic
  3. C
    simple molecular
  4. D
    giant molecular (giant covalent)
Show answer

Answer: D. A very high melting point means strong bonds throughout, and no conduction even when molten rules out ionic and metallic structures. This fits a giant covalent substance such as SiO2.

2. Graphite conducts electricity but diamond does not. Why?

  1. A
    graphite has weaker covalent bonds than diamond
  2. B
    graphite is an ionic compound
  3. C
    diamond has no electrons in its outer shells
  4. D
    in graphite each carbon uses only three electrons for bonding; the fourth is delocalised between the layers and can move
Show answer

Answer: D. In diamond every carbon uses all four outer electrons in localised σ bonds, so no electrons are free. In graphite each carbon bonds to only three others, leaving one electron per atom delocalised across the layer.

3. Which substance dissolves in water but gives a solution that does not conduct electricity?

  1. A
    sodium chloride
  2. B
    sucrose
  3. C
    hydrogen chloride
  4. D
    magnesium sulfate
Show answer

Answer: B. Sucrose is simple molecular; it dissolves by hydrogen bonding with water but forms no ions. NaCl and MgSO4 release ions, and HCl ionises in water to H+ and Cl−.

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