4. Atomic Structure

4. Atomic Structure

1. Positively charged nucleus (which contains neutrons and protons) surrounded by negatively charged electrons

2. Typical radius of an atom: 1 × 10−10 metres

   • Most (nearly all) the mass of the atom is concentrated at the nucleus
   • And the radius of the nucleus is 10 000 times smaller

3. Electron Arrangement

   • Electrons lie at different distances from the nucleus (different energy levels). The electron arrangements may change with the interaction with EM radiation

4. Isotopes and Elements

   • All atoms of the same element have the same number of protons
   • Neutral atoms have the same number of electrons and protons
   • Isotopes are atoms of the same element, but with different masses, which have the same number of protons but different number of neutrons.

5. Atoms and EM Radiation

   • When electrons change orbit (move closer or further from the nucleus)
     • When electrons move to a higher orbit (further from the nucleus)
       • The atom has absorbed EM radiation
     • When the electrons falls to a lower orbit (closer to the nucleus)
       • The atoms has emitted EM radiation
   • If an electron gains enough energy, it can leave the atom to form a positive ion.

6. How and why the atomic model has changed over time

   • 1800 - Dalton said everything was made of tiny spheres (atoms) that could not be divided
   • 1897 - JJ Thomson discovered the electron The Plum Pudding Model was formed
   • 1911 - Rutherford realised most of the atom was empty space
     • • This experiment was carried out by Geiger and Marsden
   • 1913 – Bohr produced the final model of the atom
   • Later on
     • Positive charge of nucleus could be subdivided into smaller particles, each with the same amount of charge – the proton
     • 20 years after the ‘nucleus’ was an accepted scientific idea, James Chadwick provided evidence to prove neutrons existed

7. Radiation

   • Some atomic nuclei are unstable. The nucleus gives out radiation as it changes to become more stable. This is a random process called radioactive decay.
   • Activity is the rate at which a source of unstable nuclei decays
     • So a sample with high activity has a fast rate of decay
     • Measured in Becquerel, Bq
   • Count-rate is the number of decays recorded by a detector per second
     • E.g. a Geiger-Muller Tube
   • Forms of decay
     • Alpha α (a helium nucleus)
       • Weakly penetrating (~5cm of air)
       • Highly ionising
     • Beta Minus β (electron)
       • Medium ionising
       • Medium penetration (~50cm of air, sheet of paper)
     • Gamma γ (radiation)
       • Low ionising
       • Highly penetration (very far in air, few cm of lead)
     • Neutrons

8. Nuclear Equations - Nuclear equations are used to represent radioactive decay

   • The emission of the different types of nuclear radiation may cause a change in the mass and /or the charge of the nucleus.

9. Half Life

10. Net Decline

    • Calculate the ratio of net decline of radioactive nuclei after X half-lives
      • Half the initial number of nuclei, and keep doing so X number of times
        • Net Decline = (Initial Number - Number after x Half Lives)/Initial Number

11. Contamination

    • Contamination
      • Lasts for a long period of time
      • The source of the radiation is transferred to an object
        • Radioactive contamination is the unwanted presence of radioactive atoms on other materials – the hazard is the decaying of the contaminated atoms releasing radiation
      • E.g. radioactive dust settling on your skin (your skin becomes contaminated)
    • Irradiation
      • Lasts only for a short period of time
      • The source emits radiation, which reaches the object
        • Exposing an object to nuclear radiation, but does not make it radioactive
      • E.g. radioactive dust emitting beta radiation, which “irradiates” your skin
      • Medical items are irradiated sometimes to kill bacteria on its surface, but not to make the medical tools themselves radioactive

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