Rutherford Gold Foil Experiment

Jun 20 2026

Aim: To study the scattering of alpha particles by a thin gold foil and to understand the nuclear model of the atom proposed by Ernest Rutherford.

Apparatus / Components

A radioactive source of alpha particles (e.g., Bismuth-214 or Polonium)
Lead blocks with a narrow slit (to collimate the alpha beam)
An extremely thin gold foil (about 100 nm thick)
A circular fluorescent zinc sulfide (ZnS) screen
A microscope (for viewing scintillations)
An evacuated chamber

Theory & Principle

Before this experiment, the accepted model of the atom was J.J. Thomson's "Plum Pudding" model, which suggested that positive charge and mass were spread uniformly throughout the atom. Rutherford, along with Geiger and Marsden, tested this by firing high-energy, positively charged alpha particles (He²⁺ ions) at a thin gold foil.

According to Thomson's model, the alpha particles should have passed straight through with very little deflection, as the uniform positive charge would not be concentrated enough to exert a strong repulsive force.

Actual Observations:

Most alpha particles (over 99%) passed straight through the foil undeflected.
A small fraction of particles were deflected by small angles.
Very few particles (about 1 in 20,000) rebounded completely, deflecting by angles greater than 90° (some almost 180°).

Conclusions & Rutherford's Atomic Model

Based on these unexpected results, Rutherford drew the following groundbreaking conclusions:

Empty Space: Since most particles passed through undeflected, most of the space inside an atom is empty.
Dense, Positive Center: The massive deflections and rebounds meant the positive charge and almost all the mass of the atom are concentrated in an extremely small, dense region at the center, which he named the nucleus.
Electrons: Electrons revolve around the nucleus in circular paths, much like planets orbiting the sun, leaving vast empty space between them and the nucleus.

Significance (Result)

This experiment disproved the Plum Pudding model and led to the establishment of the Nuclear Model of the atom, forming the foundation of modern nuclear physics.

Precautions (Historical Context)

The experiment must be conducted in a vacuum chamber. If air were present, the alpha particles would collide with air molecules, lose energy, and scatter before reaching the foil.
The gold foil must be exceedingly thin to ensure that an alpha particle is scattered by only a single atom rather than a cluster of atoms.

Viva Questions & Answers

Q1: Why did Rutherford choose gold for the foil?

Gold is highly malleable. It can be hammered into extremely thin sheets (up to 1000 atoms thick), which was necessary to ensure the alpha particles didn't get absorbed and were only scattered by single atoms. Gold also has a massive nucleus, providing a strong repulsive force.

Q2: What are alpha particles?

Alpha particles are helium nuclei consisting of two protons and two neutrons. They carry a charge of +2e and have a mass roughly four times that of a proton.

Q3: Why was the entire apparatus placed in a vacuum?

To prevent the alpha particles from colliding with air molecules, which would cause unwanted scattering and loss of kinetic energy before hitting the gold foil.

Q4: What was the purpose of the Zinc Sulfide (ZnS) screen?

Alpha particles are invisible. When they strike a ZnS screen, they produce tiny flashes of light (scintillations), which can be counted under a microscope to determine the angle of deflection.