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GCSE Chemistry Revision

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GCSE Chemistry revision

Chemical bonds

Chemical bonds, ionic, covalent and metallic

AQA 4.2.1.1 Foundation & Higher
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Revision summary

Key knowledge

Read on screen, then print for Cornell-style active revision.

What Are Covalent Bonds?

  • Covalent bonds form when non-metal atoms share electrons so that all atoms achieve full outer shells.
  • Covalent bonds are very strong, meaning a large amount of energy is required to break the bonds between atoms directly.

Simple Molecular Substances

  • Simple molecular substances consist of small molecules made up of just a few covalently bonded atoms, such as chlorine (Cl₂) or ammonia (NH₃).
  • To melt or boil a simple molecular substance, you only need to break the weak intermolecular forces between molecules, not the strong covalent bonds within them.
  • Because intermolecular forces are weak, simple molecular substances have low melting and boiling points — for example, chlorine boils at −34°C.

Intermolecular Forces and Molecular Size

  • Intermolecular forces are the weak forces of attraction that exist between separate molecules.
  • Larger molecules have more intermolecular forces, so more energy is needed to overcome them, resulting in higher melting and boiling points.
  • Comparing the halogens: chlorine, bromine, and iodine increase in molecular size down the group, so their boiling points increase (bromine boils at 59°C, iodine at 184°C).

Electrical Conductivity of Simple Molecular Substances

  • Simple molecular substances do not conduct electricity because they contain no free electrons and the molecules themselves carry no electric charge.
  • For a substance to conduct electricity, it must contain electrons or ions that are free to move.

Giant Covalent Structures — Overview

  • Giant covalent structures are made of huge numbers of non-metal atoms all bonded together by strong covalent bonds in a regular repeating lattice.
  • The three key examples of giant covalent structures are diamond, graphite, and silicon dioxide (silica).
  • Because all atoms are held together by strong covalent bonds throughout the entire structure, giant covalent structures have very high melting and boiling points.

Electrical Conductivity of Giant Covalent Structures

  • Giant covalent structures generally do not conduct electricity because they contain no charged particles, even when molten.
  • Graphite is an important exception, as it can conduct electricity — this is explored in more detail when studying diamond and graphite separately.

Silicon Dioxide (Silica)

  • Silicon dioxide is a giant covalent structure made of silicon and oxygen atoms in a ratio of 1:2.
  • It is also known as silica and is the main component of sand.
  • You need to be able to recognise the structure of silicon dioxide as a giant covalent lattice, though you do not need to draw it from memory.

Comparing Simple Molecular and Giant Covalent Structures

  • Simple molecular substances have low melting and boiling points because only weak intermolecular forces need to be broken, not covalent bonds.
  • Giant covalent structures have very high melting and boiling points because all atoms are joined by strong covalent bonds throughout the entire lattice.
  • Neither simple molecular substances nor most giant covalent structures conduct electricity due to the absence of free electrons or ions.