Radioactivity

• What is the structure of an atom?

  An atom consists of a positively charged nucleus surrounded by negatively charged electrons.

  
• What are the components of the nucleus of an atom?

  The nucleus is made of positive protons and neutral neutrons.

  
• How do electrons orbit the nucleus?

  Electrons orbit the nucleus at different fixed distances.
• How does the size of the nuclear radius compare to the size of the atom?

  The nuclear radius is much smaller than the radius of the atom.
• Where does most of the mass of an atom lie?

  Almost all the mass of the atom lies in the nucleus.
• What are the relative masses and charges of subatomic particles?

  • Proton: Relative Mass = 1, Relative Charge = +1
  • Neutron: Relative Mass = 1, Relative Charge = 0
  • Electron: Relative Mass = 0.0005, Relative Charge = -1
  • Positron: Relative Mass = 0.0005, Relative Charge = +1
• What is the approximate size of an atom?

  The size of an atom is approximately \(0.1 \text{ nanometers}\) or \(10^{-10} \text{ meters}\).
• What defines isotopes of an element?

  Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.
• What is an example of isotopes of carbon?


  Carbon-12, Carbon-13, and Carbon-14 are examples of isotopes of carbon.
• What do the symbols \(X\), \(A\), \(Z\), and \(N\) represent in isotopes?

  • \(X\): Symbol of the element
  • \(A\): Mass number (number of protons and neutrons)
  • \(Z\): Proton number
  • \(N\): Charge of the atom

  
• How does the charge of a neutral atom relate to its protons and electrons?

  A neutral atom has the same number of electrons and protons, resulting in no overall charge.
• What happens when electrons change orbits in an atom?

  When electrons move to a higher orbit, the atom absorbs EM radiation; when they fall to a lower orbit, the atom emits EM radiation.
• What can happen if an electron gains enough energy?

  If an electron gains enough energy, it can leave the atom and form an ion.
• What is the nature of radioactive decay?

  Decay occurs in a random process.
• What are the different forms of radioactive decay?

  • Alpha decay: Helium nucleus, highly ionising, weakly penetrating
  • Beta Minus decay: Electron, medium ionising, medium penetration
  • Beta Plus decay: Positron, medium ionising, medium penetration
  • Gamma decay: Electromagnetic radiation, low ionising, highly penetrating
  • Neutron decay

  
• What is background radiation?

  Background radiation is weak radiation that can be detected from natural or external sources.
• What are some examples of background radiation?

  • Cosmic rays
  • Radiation from underground rocks
  • Nuclear fallout
  • Medical rays

  
• How is radioactivity measured?

  • Photographic film: Darkens with radiation exposure
  • Geiger-Muller Tube: Detects radiation and produces clicks based on radiation frequency

  
• Who proposed that everything was made of atoms in 1800?

  Dalton proposed that everything was made of atoms in 1800.
• What did JJ Thomson discover in 1897?

  JJ Thomson discovered the electron in 1897.
• What was the Plum Pudding Model?

  • Proposed by JJ Thomson
  • Atoms consist of a positive "pudding" with negative electrons dispersed throughout
• What did Rutherford realize about the atom in 1911?

  Rutherford realized that most of the atom was empty space.
• What experiment did Geiger and Marsden conduct?

  Geiger and Marsden conducted the Gold Foil Experiment.
• What were the findings of the Gold Foil Experiment?

  • Most particles went straight through, indicating empty space
  • Some alpha particles were slightly deflected, indicating a positive nucleus
  • Few alpha particles were deflected by >90°, indicating that the nucleus contains most of the mass

  
• What model did Bohr produce in 1913?

  Bohr produced the final model of the atom in 1913.
• What is Beta-Minus Decay?

  • A neutron becomes a proton
  • An electron is released
• What is Beta-Plus Decay?

  • A proton becomes a neutron
  • A positron is released
• What does the equation \(Z_X^A \rightarrow Z'_Y^{A'} + \text{decay particle}\) represent?

  This equation represents a general decay process in nuclear reactions.
• What is Alpha decay?

  • An alpha particle is equivalent to a helium nucleus
  • The equation is \(Z_X^A \rightarrow Z-2_Y^{A-4} + \alpha\)
• What is Beta decay?

  • A beta particle is an electron emitted from the nucleus
  • The equation is \(Z_X^A \rightarrow Y_{Z+1}^A + e^{-} + \bar{\nu}_e\)
• What is Gamma decay?

  • A gamma ray is electromagnetic radiation
  • The equation is \(Z_X^A \rightarrow Z'_Y^{A'} + \gamma\)
• What happens to nuclei after decay?

  Nuclei after decay often have excess energy, which they release as gamma radiation during nuclear arrangement.
• What is activity in the context of radioactivity?

  Activity is the number of decays in a sample per second.
• How does activity change over time?

  Activity decreases exponentially over time.
• What are the units of activity?

  The units of activity are Becquerel (Bq).
• What is half-life in the context of radioactivity?

  • The half-life of an isotope is the time taken for half the nuclei in a sample to decay
  • It is also the time taken for the activity of a sample to decay by half
• Why can't the decay of a single nucleus be predicted?

  The decay of a single nucleus cannot be predicted, but the half-life allows for predictions about a large number of nuclei.
• How do you calculate net decline of radioactive nuclei after X half-lives?

  • Work out the ratio of net decline:
  • \( \text{net decline} = \frac{\text{initial number} - \text{number after X half-lives}}{\text{initial number}} \)
• If there were initially 80 nuclei with a half-life of 15 minutes, what is the net decline after 3 half-lives?

  The net decline is \( \frac{80 - 10}{80} = \frac{7}{8} \) or 87.5%.
• What are some uses of radioactivity?

  • Smoke alarms: Americium as an alpha emitter
  • Irradiating food: Gamma rays kill bacteria
  • Sterilisation of equipment: Gamma radiation kills microbes
  • Tracing and gauging thickness: Beta radiation measures thickness
  • Diagnosis and treatment of cancer: Gamma emitters reveal tumors and treat cancer