9478 / 2027
Nuclear Physics overview

Lesson 1 of 10

Evidence for the nucleus

  • I can infer the existence and small size of the nucleus from Rutherford alpha-scattering observations.Syllabus

    Syllabus 9478, 20(a). Infer the existence and small size of the nucleus from Rutherford alpha-scattering observations.

  • I can distinguish nucleon number from proton number.Syllabus

    Syllabus 9478, 20(b). Distinguish nucleon number from proton number.

  • I can explain isotopes through different neutron numbers and use nuclide notation.Syllabus

    Syllabus 9478, 20(c). Explain isotopes through different neutron numbers and use nuclide notation.

Make a guess. You are not marked.

In the alpha-scattering experiment, a very few alpha particles bounced almost straight back from thin gold foil. What did this show?

  1. The gold atoms are packed so tightly that alpha particles cannot pass.
  2. The positive charge and most of the mass are in a tiny central nucleus.
  3. Electrons in the gold atoms are heavy enough to reflect alpha particles.
Show the answer

The positive charge and most of the mass are in a tiny central nucleus.

Only a very dense, positively charged centre could turn a fast alpha particle right round. That so few did shows the nucleus is tiny compared with the atom.

Rutherford scattering connects a pattern of observations to an atomic model. Most alpha particles pass almost straight through a thin foil; rare large deflections reveal a small concentration of positive charge and mass.

A narrow beam of positively charged alpha particles encounters a thin metal foil, and a detecting screen records their outgoing directions. Distinguish the observed deflection pattern from the explanation inferred from it.

A narrow beam, a thin foil and a detecting screen

A narrow beam, a thin foil and a detecting screenA shielded alpha source at the left sends a narrow beam through a collimating slit towards a thin foil. A curved detecting screen surrounds much of the foil, with an opening for the incident beam. Dashed radial guides identify forward, sideways and backward detector positions; they indicate observation directions, not a claim that alpha trajectories kink at the foil. Most detected alpha particles have small deflection; a small fraction have large deflection and very few return towards the source. Source, foil and screen are schematic apparatus sizes. The following enlarged nucleus view has a separate scale.αSourceSlitThin foilDetecting screenGuides mark observation directions.Apparatus is schematic.

The screen records where particles arrive. Most have little deflection; a small fraction turn through large angles. The selected direction guides do not represent the relative number of events or the detailed path near a nucleus.

Enlarged view: repulsion bends the paths continuously

Enlarged view: repulsion bends the paths continuouslyFour selected model alpha trajectories enter from the left with rightward initial motion and pass a positive nucleus at the centre. Their acceleration is everywhere radially outward. None touches the symbolic nuclear surface. The near-central path turns continuously and emerges towards the left; more distant paths turn less. The horizontal and vertical distances share one scale. This enlarged comparison does not represent atom-to-nucleus sizes or measured scattering frequencies, and adds no required scattering formula.Selected enlarged pathsIncident motion is initially rightward.+Positive nucleusNo hard-ball impact is drawn.Paths are not a frequency sample.

A positive alpha particle is repelled by concentrated positive charge. These smooth paths show the mechanism of deflection. The symbolic nucleus and the surrounding atom are not drawn on one size scale, and four selected paths cannot indicate how often each deflection occurs.

The first view identifies the source, narrow beam, foil and detecting screen. The second is an enlarged view of continuous repulsive paths near a positive nucleus. The selected paths illustrate directions, not the relative numbers of events; the nucleus and whole atom are not shown on one size scale.
Scattering observations and the nuclear model they support
ObservationInference
Most alpha particles have little deflection.Most of the atom's volume does not contain the concentrated positive charge and mass.
A small fraction have large deflections; very few scatter backwards.A close approach to a small, massive, positive nucleus can strongly repel an alpha particle.

Large deflections are not explained by alpha particles striking individual light electrons. Nor does this experiment by itself identify the neutron or establish all electronic energy levels. Its central result is the existence of a nucleus much smaller than the whole atom.

Read nuclide notation

A nucleus is represented by . The nucleon number A, also called mass number, counts protons plus neutrons. The proton number Z, also called atomic number, counts protons and identifies the element.

Protons = Z
Neutron number N = A - Z
Electrons in a neutral atom = Z

Here N denotes the neutron number of one nucleus. When N later denotes a sample population, the stated context changes its meaning.

Isotopes have the same Z but different neutron numbers. For example, chlorine-35 and chlorine-37 both have 17 protons, but contain 18 and 20 neutrons respectively.

Worked isotope and ion count

Changing electrons does not change the isotope

A neutral atom has 17 protons, 20 neutrons and 17 electrons. Losing one electron produces a singly positive ion with 16 electrons. Its nucleus still has A = 37 and Z = 17.

Do not subtract an electron from the nuclear proton number. A is a particle count, not an exact measured mass of A atomic mass units.

For a sample population, N = (m/M)NA, with sample mass m and supplied molar mass M in matching units. NA = 6.02 × 1023 mol-1; a few times 10-4 mol therefore contain order 1020 entities. Name those entities: one atom has one nucleus, but usually many nucleons. The particle-counting explanation gives the mole and relative-mass foundations.

Check your understanding

Most alpha particles pass through a thin foil with little deflection, while very few are scattered through large angles or backwards. Which inference is supported?

  1. Large deflections are caused by collisions with individual electrons carrying most of the atomic mass.
  2. The positive nucleus fills most of the atom, and the experiment directly identifies its neutrons.
  3. Positive charge and most atomic mass are concentrated in a small nucleus.
Show the answer

Positive charge and most atomic mass are concentrated in a small nucleus.

Most paths avoid a close approach to the small concentration, while a rare close approach to its positive charge can strongly repel an alpha particle. The observations support a nucleus much smaller than the whole atom.