Radioactivity Summary Notes

Atomic Structure

  • Atomic Structure: Fundamental particles (protons, neutrons, electrons) and isotopes (atoms of same element, different neutron numbers). The atomic number (Z) is the number of protons, and the mass number (A) is the total number of protons and neutrons (nucleons). Isotopic forms are key to understanding radioactive decay.

Discovering Atomic Structure

  • Plum Pudding Model: Thomson (1904) proposed atoms were a sphere of positive charge with scattered negative electrons.

  • Rutherford’s Alpha Scattering: (1914) bombarded gold foil with alpha particles.

    • Observations: Most passed through, some deflected, a few greatly deflected.

    • Conclusions: Atoms are mostly empty space with a dense, positively charged nucleus.

Particle Symbols and Charges

  • Particles: Proton (p) with +1 charge, Neutron (n) with 0 charge, Electron (e-) with -1 charge (orbits nucleus).

  • Nucleons: Protons and neutrons in the nucleus. The mass number (A) represents the total number of nucleons: A = Z + N (where Z is atomic number/protons, N is neutron number).

  • Isotopes: Atoms of the same element (same Z) but with different numbers of neutrons (different N and thus different A).

Bohr’s Model

  • Proposed by Niels Bohr: Electrons orbit the nucleus at fixed energy levels. Electrons absorb or emit energy to transition between these levels.

Nuclear Radiation

  • Radioactive Decay: A random process where unstable atomic nuclei spontaneously emit radiation to become more stable.

  • Types of Radiation: Their nature, mass, and charge are crucial for the Edexcel specification.

    • Alpha (α): Helium nuclei (2 protons, 2 neutrons), mass = 4, charge = +2.

    • Beta-minus (β-): Fast-moving electron, mass ≈ 0 (negligible), charge = -1.

    • Beta-plus (β+): Positron (antimatter electron), mass ≈ 0, charge = +1.

    • Gamma (γ): High-energy electromagnetic wave (photon), mass = 0, charge = 0.

Dangers of Nuclear Radiation

  • Ionising abilities and penetration ability (key for safety protocols):

    • Alpha: Highly ionising, very low penetration. Stopped by a few cm of air, paper, or skin.

    • Beta: Moderately ionising, medium penetration. Stopped by a few mm of aluminum, clothing. Can penetrate skin.

    • Gamma: Weakly ionising, very high penetration. Requires thick lead or concrete to significantly reduce intensity. Can pass through the body.

Detection and Background Radiation

  • Detection Methods: Photographic film (darkens due to exposure) and Geiger-Müller counters (detect ionising radiation).

  • Sources of Background Radiation: Naturally occurring and man-made sources. Key examples include:

    • Natural: Radon gas from rocks, cosmic rays from space, radioactive isotopes in food/drinks and rocks/soil.

    • Man-made: Medical applications (X-rays, radiotherapy), nuclear weapon fallout, nuclear power generation.

Nuclear Decay

  • Processes: Understanding how mass number (A) and atomic number (Z) change during decay is vital for nuclear equations.

    • Alpha decay: Nucleus emits an alpha particle. Atomic number decreases by 2, mass number decreases by 4. {_Z^A X \rightarrow _{Z-2}^{A-4} Y + 2^4 \alpha} (or {2^4 He})

    • Beta-minus decay: A neutron in the nucleus transforms into a proton, emitting an electron (beta-minus particle). Atomic number increases by 1, mass number remains unchanged. {_Z^A X \rightarrow _{Z+1}^A Y + {-1}^0 e} (or {{-1}^0 \beta})

    • Beta-plus decay: A proton in the nucleus transforms into a neutron, emitting a positron (beta-plus particle). Atomic number decreases by 1, mass number remains unchanged. {_Z^A X \rightarrow _{Z-1}^A Y + {+1}^0 e} (or {{+1}^0 \beta})

    • Gamma radiation: Occurs when a nucleus de-excites, releasing high-energy photons. No change in atomic number or mass number, as it's pure energy emission. Often accompanies alpha or beta decay.

Activity and Half-life

  • Half-life (T_{1/2}): The average time taken for half of the radioactive nuclei in a sample to decay, or for the activity of a sample to halve. It is constant for a given isotope.

  • Activity: The rate at which unstable nuclei decay in a sample. Measured in Becquerels (Bq), where 1 Bq = 1 decay per second. Activity decreases exponentially over time.

  • Decay Types: Activity calculations often involve accounting for background radiation: C{actual} = C{measured} - C_{background}. Interpreting decay curves for half-life determination is also a key skill.