atoms and isotopes (p4)

• Mass number

  Number of protons + neutrons in an atom
• Atomic number

  Number of protons in an atom
• Isotopes

  Atoms of an element have the same atomic number but different mass numbers
• Atom

  • Smallest part of an element that can exist
  • Has protons, neutrons and electrons
• Nucleus

  • Center of an atom
  • Contains protons and neutrons
• Electron

  • Orbits the nucleus at different distances (energy levels)
  • Can move to higher or lower energy levels
• Electron absorbing electromagnetic radiation

  Moves to a higher energy level further from the nucleus
• Electron emitting electromagnetic radiation

  Moves to a lower energy level closer to the nucleus
• Dalton's atomic model

  • Atoms were solid spheres
• Thompson's atomic model

  • Discovered the electron
  • plum pudding model
• Bohr's atomic model

  • Electrons orbit the nucleus in fixed shells
• Chadwick's discovery


  • Discovered the neutron
• Radioactive decay

  Process where unstable atomic nuclei release radiation to become more stable
• Activity

  Rate at which a radioactive substance decays, measured in Becquerels
• Count rate


  Decays detected per second, measured with a Geiger-Muller tube
• Types of nuclear radiation

  • Alpha particles
  • Beta particles
  • Gamma rays
• Alpha particles

  • Made up of 2 neutrons and 2 protons
  • Have low penetrating power but are strongly ionising
• Beta particles

  • High speed electrons ejected from the nucleus
  • Have moderate penetrating power and ionising ability
• Gamma rays

  • Electromagnetic radiation
  • Have high penetrating power but low ionising ability
• Radioactive decay is a random process
• Half-life

  Time it takes for the number of undecayed nuclei or activity of a radioactive sample to fall to half its initial value
• Calculating radioactive decay

  Ratio of the decline = (Initial number - Number after) / Initial number
• Radioactive contamination

  When radioactive particles come into direct contact with material or living cells, making them radioactive
• Irradiation

  When an object or person is exposed to radiation but does not become radioactive
• Effects of different types of radiation

  • Alpha - High ionisation, low penetration
  • Beta - Moderate ionisation, moderate penetration
  • Gamma - Low ionisation, high penetration
• Background radiation

  Radiation present in the environment from cosmic rays, underground rocks, nuclear fallout, and medical uses
• Medical uses of radioactive isotopes

  • Technetium as a medical tracer
  • Gamma emitters used in chemotherapy
• Nuclear fission

  The splitting of large, unstable nuclei into smaller nuclei, releasing energy and neutrons
• Spontaneous fission is rare, usually fission requires the nucleus to first absorb a neutron
• Nuclear fission process

  1. Unstable nucleus absorbs a neutron

  2. Nucleus splits into smaller nuclei

  3. 2/3 neutrons and gamma rays are emitted

  4. Energy is released
• The chain reaction in a nuclear reactor is controlled, while the chain reaction in a nuclear weapon is uncontrolled
• how do atoms form positive ions

  sometimes radition cause electrons to be removed from an atom through ionisation
• Ernest Rutherford 


  -alpha particle scattering experiment
• what were the observations of the alpha particle experiment

  most particles passed straight through

  -atom is empty space

  -mass is concentrated in the centre

  some particles were deflected

  -nucleus had positive charge
• alpha radiation in smoke detectors

  alpha particles ionise air particles creating an electric current, when smoke interrupts the flow of ions, the current drops , triggering alarm
• beta particles in material thickness measurement

  can penetrate materials to gain information
• gamma rays as medical tracers 


  injected into blood and studied, ability to penetrate tisues without being absorbed, low ionising power
• nuclear fusion

  -joining of two light nuclie to form a heavier nucleus

  -releases lots of energy

  -some of the mass may be converted into the energy of radiation
• Rutherford 


  realised the atom was mostly empty space

  gold foil experiment

  • most particles went straight through - empty space
  • slightly defelcted - nuclues must be charged
  • deflected less than 90 degress - contained most of mass