IGCSE Physics Revision Notes: Atomic Physics

IGCSE Physics Revision Notes: Atomic Physics

5. Atomic Physics

5.1 The Nuclear Atom
5.1.1 Atomic Model

  • Definition of an atom: An atom consists of a small, dense, positively charged nucleus that is surrounded by negatively charged electrons.

  • Size comparison: The atom is approximately 100,000 times larger than the nucleus.

5.1.2 Nucleus

  • Composition of the nucleus: Atoms are composed of three fundamental particles—protons, neutrons (found in the nucleus), and electrons (surrounding the nucleus).

  • Atomic symbol representation:

    • The top number (nucleon number, A): Represents the total number of particles (protons + neutrons) in the nucleus.

    • The bottom number (proton number, Z): Represents the total number of protons in the nucleus.

  • Proton count: The number of protons is equal to the proton number.

  • Electron count: In a neutral atom, the number of electrons equals the number of protons.

  • Neutron calculation: The number of neutrons can be calculated as follows:

    • Neutrons=A−ZNeutrons = A - Z

  • Terminology:

    • Nucleon: A particle in the nucleus (either proton or neutron).

    • Nuclide: A nucleus with a specific combination of protons and neutrons.

5.1.3 Nuclear Reactions

  • Nuclear fission: This is a process where large, unstable nuclei, like Uranium-235, break apart into two smaller nuclei and a few neutrons when struck by a neutron—this releases a significant amount of energy. The neutron initiates the fission because it has no charge and is thus not repelled by the nucleus's positive charge.

  • Nuclear fusion: This involves the combination of small nuclei (e.g. hydrogen) at high speeds to form larger nuclei, which also releases energy.

  • Nuclear equations: These equations represent nuclear reactions, similar to chemical equations. For example, a fission reaction where a Uranium nucleus is hit by a neutron can be written as follows:

    • Example nuclear equation for fission:
      236<em>92Uightarrow90</em>38Sr+144<em>54Xe+21</em>0n^{236}<em>{92}U ightarrow ^{90}</em>{38}Sr + ^{144}<em>{54}Xe + 2^{1}</em>{0}n

    • Balancing nuclear equations: Ensure that the sum of nucleon numbers and proton numbers is equal on both sides of the equation.

5.2 Radioactivity
5.2.1 Detection of Radioactivity

  • Background radiation: This is the unavoidable radiation present in the environment, originating from natural and artificial sources. Examples include radiation from cosmic sources and medical procedures like X-rays. Levels of background radiation can vary geographically.

  • Detecting radiation: When radiation encounters an atom, it can ionize it by knocking out electrons. Various detectors can identify these ions or the resultant chemical changes. Common detectors comprise:

    • Photographic film

    • Geiger-Muller (GM) tubes

    • Ionisation chambers

    • Scintillation counters

5.2.2 Characteristics of Radiation


  • Types of radiation:

    • Alpha (α) particles: Composed of 2 protons and 2 neutrons (like a helium nucleus); emitted by large unstable nuclei.

    • Beta (β) particles: High energy electrons emitted from the nucleus, typically from nuclei with excess neutrons.

    • Gamma (γ) rays: High-energy electromagnetic waves emitted from nuclei needing to lose energy.


  • Ionisation impact: Radiation can knock out electrons from atoms, leading to chemical changes and potential damage to living cells, causing mutations, or cancer.


  • Ionisation properties summary:

    Radiation Type

    Ionisation Ability

    Penetration Power


    Alpha (α)

    High

    Low


    Beta (β)

    Moderate

    Medium


    Gamma (γ)

    Low

    High

    5.2.3 Radioactive Decay

    • Radioactive decay: Unstable isotopes decay by emitting radiation in order to stabilize their nuclei.

    • Alpha emission: When an alpha particle is emitted, the atomic number decreases by 2, and the mass number decreases by 4.

      • Equation for alpha emission:
        A<em>ZXightarrowA−4</em>Z−2Y+24He^{A}<em>{Z}X ightarrow ^{A-4}</em>{Z-2}Y + ^{4}_{2}He

    • Beta emission: Occurs when a neutron in the nucleus changes into a proton, emitting a beta particle (electron). The atomic number increases by 1 while the mass number remains unchanged.

      • Equation for beta emission:
        A<em>ZXightarrowA</em>Z+1Y+−10e^{A}<em>{Z}X ightarrow ^{A}</em>{Z+1}Y + ^{0}_{-1}e

    5.2.4 Half-Life

    • Definition: The half-life of a radioactive isotope is the time required for half of the isotope’s nuclei to decay. Every time a half-life passes, the remaining activity or number of nuclei halves. Half-lives can vary significantly—from fractions of seconds to billions of years.

    • Example of calculating half-life: Assessing the decay activity from an initial measurement down to half requires plotting data on a graph and determining the intersection with the time axis based on halving activities.

    • Graphical representation: The graph typically begins high and approaches the time axis without ever actually reaching it, engendering an exponential decay curve.

    5.2.5 Safety Precautions

    • Dangers of radiation exposure: Radiation can ionize atoms leading to cell damage: mutations, cancer, or cell death are potential outcomes.

    • Safety protocols for handling radioactive materials: Include:

      • Store sources in lead-lined containers and maintain distance.

      • Limit exposure time and return sources swiftly to storage.

      • Handle sources with tongs and keep them at arm’s length.

      • Proper protective gear should be employed when necessary to prevent contamination.