Edexcel A Level (IAL) Physics-4.32 The Nuclear Model of the Atom- Study Notes- New Syllabus

Large-Angle Alpha Particle Scattering and the Nuclear Model of the Atom

Experiments involving the scattering of alpha particles provided crucial evidence that atoms have a small, dense nucleus. These results led to a major change in our understanding of atomic structure.

The Alpha Particle Scattering Experiment

In the experiment, a beam of alpha particles was directed at a very thin sheet of gold foil.

• Alpha particles are positively charged helium nuclei.
• They have relatively high mass and high kinetic energy.
• Their paths after passing through the foil were observed using a fluorescent screen.

Observations

• Most alpha particles passed straight through the foil with little or no deflection.
• Some alpha particles were deflected through small angles.
• A very small number were deflected through large angles (greater than \( 90^\circ \)).
• A tiny fraction were reflected almost straight back.

 Why Large-Angle Scattering Was Surprising

At the time, the accepted model was the plum pudding model, where:

• Positive charge was spread uniformly throughout the atom.
• Electrons were embedded within this positive “pudding”.

This model predicted:

• Only small deflections of alpha particles.
• No large-angle scattering.

The experimental results therefore contradicted the plum pudding model.

 Conclusions from the Scattering Results

• Most of the atom is empty space (explains why most alpha particles passed through).
• Positive charge and most of the mass are concentrated in a very small region.
• This central region is called the nucleus.
• The nucleus must be very dense to cause large-angle deflections.

This led to the nuclear model of the atom.

The Nuclear Model of the Atom

• A tiny, dense, positively charged nucleus at the centre.
• Electrons move around the nucleus.
• The atom is mostly empty space.

This model successfully explained large-angle alpha particle scattering.

Development of Atomic Models Over Time   

(a) Dalton’s Model

• Atoms are solid, indivisible spheres.
• No internal structure.

(b) Thomson’s Plum Pudding Model

• Electrons embedded in a diffuse positive charge.
• No nucleus.

(c) Rutherford’s Nuclear Model

• Small, dense, positively charged nucleus.
• Most of atom is empty space.

(d) Bohr Model (Later Improvement)

• Electrons orbit the nucleus in fixed energy levels.
• Explains atomic line spectra.

Why Large-Angle Scattering Is Key Evidence

• Only a concentrated positive nucleus can repel alpha particles so strongly.
• A spread-out positive charge cannot produce such deflections.
• Therefore, atoms must contain a compact nucleus.