Atoms, Nuclei and Radiation

2026 Syllabus Objectives

By the end of this topic, you should be able to:

  1. Infer from the results of the α-particle scattering experiment the existence and small size of the nucleus
  2. Describe a simple model for the nuclear atom to include protons, neutrons and orbital electrons
  3. Distinguish between nucleon number and proton number
  4. Understand that isotopes are forms of the same element with different numbers of neutrons in their nuclei
  5. Understand and use the notation A/Z X for the representation of nuclides
  6. Understand that nucleon number and charge are conserved in nuclear processes
  7. Describe the composition, mass and charge of α-, β- and γ-radiations (both β– (electrons) and β+ (positrons) are included)
  8. Understand that an antiparticle has the same mass but opposite charge to the corresponding particle, and that a positron is the antiparticle of an electron
  9. State that (electron) antineutrinos are produced during β– decay and (electron) neutrinos are produced during β+ decay
  10. Understand that α-particles have discrete energies but that β-particles have a continuous range of energies because (anti)neutrinos are emitted in β-decay
  11. Represent α- and β-decay by a radioactive decay equation of the form ²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂α
  12. Use the unified atomic mass unit (u) as a unit of mass

1. The Alpha-Particle Scattering Experiment

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Background

In the early 1900s, scientists wanted to understand what atoms looked like inside. Ernest Rutherford and his team (Geiger and Marsden) designed an experiment to probe the structure of atoms. This experiment is often called the gold foil experiment or the alpha-particle scattering experiment.

How the Experiment Worked

Setup:

  • A beam of alpha particles (α-particles) was fired at a very thin sheet of gold foil
  • The gold foil was only about 10⁻⁶ m thick (extremely thin)
  • The experiment was conducted in a vacuum chamber to prevent air molecules from interfering
  • A detector was placed around the foil to record where the alpha particles went after hitting the foil
  • The detector could be moved to different angles to measure deflection

What are alpha particles? Alpha particles are the nucleus of a helium atom. They consist of 2 protons and 2 neutrons stuck together. This means they:

  • Have a positive charge (+2e, where e is the basic unit of charge)
  • Are relatively heavy (mass = 4 atomic mass units)
  • Travel at high speeds (around 10⁶ to 10⁷ m/s)

Observations and Results

When the alpha particles hit the gold foil, three main things happened:

Observation 1: Most alpha particles went straight through

  • The vast majority of alpha particles passed through the foil without being deflected at all, or were deflected by only very small angles (less than 10°)
  • This was surprising because if atoms were solid throughout, the particles should have been blocked

Observation 2: Some alpha particles deflected at small angles

  • A small number of alpha particles were deflected through small angles
  • This suggested there was something in the atom that could push the alpha particles aside

Observation 3: Very few alpha particles bounced straight back

  • An extremely small number of alpha particles (about 1 in 8,000) deflected at angles greater than 90°
  • Some even bounced straight back toward the source
  • Rutherford said this was like firing a bullet at tissue paper and having it bounce back at you!

Conclusions: What the Results Tell Us

From these observations, Rutherford concluded:

The atom is mostly empty space

  • Because most alpha particles went straight through, the atom cannot be solid
  • There must be large empty regions inside the atom

The nucleus exists and is very small

  • The few alpha particles that bounced back must have hit something very dense and positively charged
  • This "something" is the nucleus — a tiny, dense region at the center of the atom
  • The nucleus is positively charged (which repels the positively charged alpha particles)

The nucleus contains most of the atom's mass

  • For an alpha particle to bounce back, it must collide with something much heavier than itself
  • This means the nucleus is where most of the atom's mass is concentrated

Size comparison:

  • The nucleus has a diameter of about 10⁻¹⁵ m
  • The whole atom has a diameter of about 10⁻¹⁰ m
  • This means the atom is about 100,000 times larger than the nucleus

Think of it this way: if the nucleus were the size of a marble in the center of a football stadium, the electrons would be tiny specks orbiting near the stadium walls. The space in between is mostly empty!

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