Atomic Structure

    OCR
    GCSE

    The nuclear model describes the atom as a positively charged nucleus composed of protons and neutrons, surrounded by orbiting electrons, replacing the earlier plum pudding model following Rutherford's alpha-scattering experiment. Isotopes are defined by variations in neutron number, while unstable nuclei undergo random radioactive decay via alpha, beta, or gamma emission to achieve stability. Candidates must quantify this decay using the concepts of activity and half-life, and distinguish between the hazards of irradiation and contamination in medical and industrial contexts.

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    Objectives
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    Exam Tips
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    Pitfalls
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    Key Terms
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    Mark Points

    What You Need to Demonstrate

    Key skills and knowledge for this topic

    • Award 1 mark for stating that the nucleus is positively charged and contains almost all the mass of the atom
    • Credit responses that explain the atom is mostly empty space because most alpha particles passed straight through the foil
    • Award 1 mark for defining isotopes as atoms of the same element with the same number of protons but different numbers of neutrons
    • Candidates must identify that in the Bohr model, electrons orbit at specific distances or energy levels from the nucleus
    • Award 1 mark for calculating the number of neutrons by subtracting the atomic number from the mass number

    Example Examiner Feedback

    Real feedback patterns examiners use when marking

    • "You correctly identified the subatomic particles, but check your calculation for neutrons: Mass Number - Atomic Number"
    • "Good description of the Rutherford experiment. To gain full marks, explain *why* the deflection happens (electrostatic repulsion)"
    • "You mentioned the Plum Pudding model had a nucleus — remember, it was a 'soup' of positive charge, the nucleus came later"
    • "Excellent use of standard form. Now try to express the size of the nucleus as a ratio of the total atomic radius"

    Marking Points

    Key points examiners look for in your answers

    • Award 1 mark for stating that the nucleus is positively charged and contains almost all the mass of the atom
    • Credit responses that explain the atom is mostly empty space because most alpha particles passed straight through the foil
    • Award 1 mark for defining isotopes as atoms of the same element with the same number of protons but different numbers of neutrons
    • Candidates must identify that in the Bohr model, electrons orbit at specific distances or energy levels from the nucleus
    • Award 1 mark for calculating the number of neutrons by subtracting the atomic number from the mass number

    Examiner Tips

    Expert advice for maximising your marks

    • 💡When describing the alpha scattering experiment, explicitly link the observation (e.g., large deflection) to the specific conclusion (e.g., concentrated positive mass)
    • 💡Ensure you can convert atomic dimensions (approx 1x10^-10 m) and nuclear dimensions (approx 1x10^-14 m) into standard form ratios
    • 💡For 6-mark questions on the history of the atom, structure your answer chronologically: Dalton → Thomson → Rutherford → Bohr
    • 💡Remember that atoms are neutral because the number of protons equals the number of electrons; ions are formed by electron loss/gain only

    Common Mistakes

    Pitfalls to avoid in your exam answers

    • Confusing the atomic number (protons) with the mass number (protons + neutrons) when calculating neutron count
    • Describing the Plum Pudding model as having a nucleus; candidates often forget it was a sphere of positive charge with embedded electrons
    • Stating that alpha particles 'hit' or 'bounce off' the nucleus physically, rather than being deflected by electrostatic repulsion
    • Failing to appreciate the scale difference, often drawing the nucleus disproportionately large in diagrams without annotation

    Study Guide Available

    Comprehensive revision notes & examples

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    Key Terminology

    Essential terms to know

    Evolution of the atomic model (Plum Pudding vs. Nuclear)
    Subatomic particles, isotopes, and ionisation
    Radioactive decay modes (alpha, beta, gamma, neutron)
    Activity, half-life, and decay equations
    Hazards: Irradiation versus Contamination

    Likely Command Words

    How questions on this topic are typically asked

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    Describe
    Explain
    Calculate
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