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Edexcel GCSE Physics

Atomic structure and radioactivity — Edexcel GCSE Physics revision

Free revision notes, key terms, common exam traps and 5 practice questions with answers. About 7 minutes to read.

Atomic model and structure

  • An atom has a tiny, dense, positively charged nucleus made of protons and neutrons, surrounded by electrons in shells.
  • Atoms have no overall charge because the number of protons equals the number of electrons.
  • The atomic model developed over time: from Dalton's solid spheres, to Thomson's plum pudding model, to Rutherford's nuclear model (from alpha scattering), to Bohr's model with fixed electron shells.
  • Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.

Radioactive decay and types of radiation

  • Radioactive decay is a random process where unstable nuclei emit radiation to become more stable; you cannot predict when an individual nucleus will decay.
  • Alpha particles are two protons and two neutrons (helium nucleus); they are strongly ionising but weakly penetrating, stopped by paper or a few cm of air.
  • Beta particles are high-speed electrons emitted from the nucleus when a neutron changes into a proton; moderately ionising and penetrating, stopped by a few mm of aluminium.
  • Gamma rays are electromagnetic waves from the nucleus; weakly ionising but highly penetrating, needing thick lead or concrete to significantly reduce them.
  • Nuclear equations must balance mass number (top) and atomic number (bottom) on both sides.

Key terms

Nucleus
The small, dense, positively charged centre of an atom containing protons and neutrons.
Isotope
Atoms of the same element with different numbers of neutrons.
Half-life
Time for the number of radioactive nuclei or activity of a sample to halve.
Activity
The rate at which a radioactive source decays, measured in becquerels.
Ionising radiation
Radiation that can knock electrons off atoms, creating ions.
Irradiation
Exposure to radiation from a source, without the object becoming radioactive itself.

Common exam traps

  • Confusing contamination (radioactive material present) with irradiation (exposure only).
  • Thinking half-life means all the material disappears — it means half the nuclei have decayed, some will always remain (theoretically).
  • Mixing up which radiation type is most ionising vs most penetrating (they're roughly opposite).

Practice questions with answers

  1. 1. What is found in the nucleus of an atom?

    • • Protons and electrons
    • • Protons and neutrons
    • • Neutrons and electrons
    • • Only protons

    Answer: Protons and neutrons

    The nucleus contains protons (positive) and neutrons (neutral); electrons orbit outside.

  2. 2. Isotopes of an element have the same number of:

    • • Neutrons
    • • Protons
    • • Electrons and neutrons
    • • Nucleons only

    Answer: Protons

    Isotopes share the same proton number but differ in neutron number.

  3. 3. Which scattering experiment led to the nuclear model of the atom?

    • • Thomson's cathode ray experiment
    • • Rutherford's alpha scattering experiment
    • • Bohr's spectral line experiment
    • • Dalton's gas experiment

    Answer: Rutherford's alpha scattering experiment

    Most alpha particles passed through gold foil, but some bounced back, showing a small dense positive nucleus.

  4. 4. Name the model of the atom that came before the nuclear model, where negative electrons sit in a positive 'pudding'.

    Answer: Plum pudding model

    Thomson's plum pudding model preceded Rutherford's nuclear model.

  5. 5. What is an alpha particle?

    • • A high-speed electron
    • • Two protons and two neutrons
    • • An electromagnetic wave
    • • A single neutron

    Answer: Two protons and two neutrons

    An alpha particle is equivalent to a helium nucleus (2 protons, 2 neutrons).

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