(c) Fission and fusion

7.17 know that nuclear reactions, including fission, fusion and radioactive decay, can be a source of energy

  • Nuclear Fission:

The process where heavy atoms are split into smaller, lighter atoms. This releases energy.

  • Nuclear Fission:

The process where lighter atoms are forced to join together to make heavier atoms. This releases energy.

  • Radioactive Decay:

Within the core of the Earth, radioactive isotopes of elements such as uranium, thorium and potassium provide a large proportion of the heat within the Earth through radioactive decay.

7.20 describe how a chain reaction can be set up if the neutrons produced by one fission strike other U-235 nuclei

Chain Reaction:

  • The three neutrons produced by the fission may hit other nuclei of uranium 235, causing the process to repeat.
  • For a chain reaction to occur, there is a minimum mass of uranium 235 required. This is known as the critical mass.

7.21 describe the role played by the control rods and moderator in the fission process


  • Graphite is used as a moderator.
  • The purpose of the moderator is to absorb some of the kinetic energy of the neutrons to slow them down.
  • This is because slow neutrons are more easily absorbed by uranium 235 nuclei.


Control rods:

  • Made of boron or cadmium.
  • The purpose of the control rods is to absorb neutrons and completely remove them from the fission process.
  • Helps adjust the rate of nuclear fission in the reactor.

7.26 explain why nuclear fusion does not happen at low temperatures and pressures, due to electrostatic repulsion of protons

  • For nuclear fusion to occur, very high temperatures are required to overcome the repulsive force between the positively charged nuclei of each isotope.
  • High pressures are also needed to increase the chance of fusion between the nuclei.
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Section 1: Principles of chemistry

      a) States of matter

      b) Atoms

      c) Atomic structure

     d) Relative formula masses and molar volumes of gases

     e) Chemical formulae and chemical equations

     f) Ionic compounds

     g) Covalent substances

     h) Metallic crystals

     i) Electrolysis

 Section 2: Chemistry of the elements

     a) The Periodic Table

     b) Group 1 elements: lithium, sodium and potassium

     c) Group 7 elements: chlorine, bromine and iodine

     d) Oxygen and oxides

     e) Hydrogen and water

     f) Reactivity series

     g) Tests for ions and gases

Section 3: Organic chemistry

     a) Introduction

     b) Alkanes

     c) Alkenes

     d) Ethanol

Section 4: Physical chemistry

     a) Acids, alkalis and salts

     b) Energetics

     c) Rates of reaction

     d) Equilibria

Section 5: Chemistry in industry

     a) Extraction and uses of metals

     b) Crude oil

     c) Synthetic polymers

     d) The industrial manufacture of chemicals