unstable and stable nuclei

• Keeping the nucleus stable requires a lot of effort
• Strong nuclear force

  An attractive force that’s stronger than the electrostatic force
• Strong nuclear force characteristics

  1. Must be attractive

  2. Has a very short range

  3. Strength falls beyond a few femtometres

  4. Works equally between all nucleons

  5. Must be repulsive at very small separations
• The strong nuclear force can only hold nucleons together when they’re separated by up to a few femtometres (1 fm = 1 × 10–15 m)
• The strength of the strong nuclear force quickly falls beyond a distance of about 3 fm
• The strong nuclear force works equally between all nucleons
• At very small separations, the strong nuclear force must be repulsive
• Forces acting on nucleons in a nucleus

  • Electrostatic forces
  • Gravitational forces
  • Strong nuclear force
• The repulsion from the electrostatic force is much bigger than the gravitational attraction
• If only electrostatic and gravitational forces acted in the nucleus, the nucleons would fly apart
• Alpha emission


  Only happens in very big nuclei, like uranium and radium
• The nuclei of very big atoms are too massive for the strong nuclear force to keep them stable
• Alpha particle emission

  1. Proton number decreases by 2

  2. Nucleon number decreases by 4
• Alpha particles have a very short range — only a few cm in air
• Tracks left by alpha particles can be observed in a cloud chamber
• A Geiger counter can measure the amount of ionising radiation
• The strong nuclear force can be plotted on a graph to show how it changes with the distance of separation between nucleons
• Strong nuclear force characteristics with nucleon separation


  1. Repulsive for very small separations

  2. Attractive after about 0.5 fm to 3fm

  3. falls rapidly towards zero after about 3 fm (repulsion)
• The electrostatic repulsive force extends over a much larger range
• Strong interaction

  Another name for the strong nuclear force
• The strong nuclear force binds the nucleus together
• Beta-minus decay is the emission of an electron from the nucleus along with an antineutrino
• Beta decay


  Happens in isotopes that are unstable due to being ‘neutron rich’
• Beta decay process

  1. Neutron changes into a proton

  2. Proton number increases by one

  3. Nucleon number stays the same
• Observations showed that the energy of the particles after beta decay was less than it was before
• Wolfgang Pauli suggested another particle was being emitted during beta decay
• The particle suggested by Pauli had to be neutral and have zero or almost zero mass
• The neutrino was eventually observed 25 years later, providing evidence for Pauli’s hypothesis
• Energy, momentum, charge and nucleon number are conserved in every nuclear reaction
• In beta decay, a tiny neutral particle called an antineutrino is released
• The antineutrino carries away some energy and momentum
• Beta particles have a much greater range than alpha particles
• Scientists originally thought that the only particle emitted from the nucleus during beta decay was an electron
• how does the strong nuclear force keep nucleus stable 


  The Strong Nuclear Force Binds Nucleons Together by counteracting the EM force of repulsion between the proton and neutron. ( keeps the proton and neutron together)
• strong nuclear force only acts on nucleons
• what is a unstable nuclei


  the strong force cannot hold the nucleus together. theres too much energy.
• alpha decay


  2 neutrons and 2 protons emitted by unstable nuclei
• gamma radiation


  passes through thick metal, no mass or charge, nucleus turns stable after this is emitted
• beta decay


  happens via weak force