5.1 The nuclear model of the atom

5.1.1 The atom

IGCSE Physics: Nuclear Model of the Atom

Core Content

1. Structure of an Atom

  • An atom consists of:
    • A positively charged nucleus (containing protons and neutrons)
    • Negatively charged electrons orbiting the nucleus in energy levels (shells)
  • The nucleus is tiny compared to the atom but contains most of its mass
  • The atom is mostly empty space between the nucleus and electrons
The nuclear model of the atom

Figure 1: The nuclear model of the atom

2. Formation of Ions

  • Positive ions form when atoms lose electrons
    Na → Na⁺ + e⁻

    Shows a sodium atom losing one electron to become a positively charged sodium ion.

  • Negative ions form when atoms gain electrons
    Cl + e⁻ → Cl⁻

    Chlorine atom gaining one electron to become a negatively charged chloride ion.

  • Ions are charged because the number of protons ≠ electrons

Supplement Content

3. Alpha Particle Scattering Experiment (Rutherford's Gold Foil Experiment)

  • Procedure:
    • Alpha particles (positively charged) were fired at a thin gold foil
    • A detector measured the deflection angles of the particles
Experimental setup of alpha particles striking gold foil

Figure 2: Experimental setup of alpha particles striking gold foil, showing deflections

  • Observations:
    • Most alpha particles passed straight through (Figure 3)
    • A few were slightly deflected
    • Very few (1 in 8000) bounced back
  • Conclusions (Evidence for the Nuclear Model):
    • (a) Mostly empty space: Most alpha particles passed through because atoms are not densely packed (unlike the plum pudding model)
    • (b) Concentrated mass: The few deflections showed a tiny, dense nucleus containing most of the atom's mass
    • (c) Positive nucleus: Rebounded alpha particles indicated a positively charged nucleus repelling them
Alpha particles passing through empty space

Figure 3: Alpha particles passing through empty space, with a few deflected near the nucleus

Key Notes

  • Scale of the Atom:
    • Atom diameter: ~10⁻¹⁰ m
    • Nucleus diameter: ~10⁻¹⁵ m (100,000 times smaller)
IGCSE Physics Syllabus Reference: 5.1 The nuclear model of the atom

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