In the early 20th century, the scientific community began to look inside the atom. Ernest Rutherford, a physicist working at the University of Manchester, stood at the forefront of this investigation into the atom’s internal architecture. His goal was to use newly discovered radioactive particles as high-speed probes to map the distribution of matter and charge within the atom. The famous gold foil experiment would ultimately shatter the established understanding of the atom and lay the groundwork for modern nuclear physics.
The Plum Pudding Theory
Before Rutherford’s work, the prevailing scientific model of the atom was the “plum pudding” theory, proposed by J.J. Thomson after his discovery of the electron in 1897. This model envisioned the atom as a uniform sphere of positive charge. Negatively charged electrons were thought to be distributed throughout this sphere.
The model was designed to account for two known facts: the existence of negative electrons and the overall electrical neutrality of the atom. Thomson posited that the diffuse, low-density positive charge balanced the negative charges of the embedded electrons. This configuration led to a specific prediction for how high-energy particles would interact with the atom’s matter.
Scientists expected that any small, fast-moving particle fired at an atom would pass straight through the relatively weak, uniform electrical field of the positive “pudding” mass. Because the atom’s mass and charge were believed to be spread out, the incoming particle would experience only minor deflections. The Plum Pudding Model suggested the atom was a soft, penetrable structure, which was the expectation Rutherford and his team held when they began their experimental work.
Designing the Experiment and Unexpected Observations
Rutherford directed his associates, Hans Geiger and Ernest Marsden, to execute an experiment using alpha particles as atomic projectiles. Alpha particles, which are positively charged helium nuclei, were emitted from a radioactive source and directed at a target of extremely thin gold foil. The gold was chosen because it could be hammered down to a thickness of only a few hundred atoms, ensuring the alpha particles interacted with individual atoms.
A circular screen coated with zinc sulfide was placed around the gold foil to detect the path of the alpha particles. When an alpha particle struck the screen, a tiny flash of light (scintillation) was produced, allowing the researchers to track the particles’ final trajectories. Based on the accepted atomic model, they anticipated that the vast majority of the alpha particles would pass through the foil with little to no change in direction, producing flashes directly opposite the source.
The experimental observations largely confirmed the expectation, as nearly all of the alpha particles—over 99%—did indeed pass straight through the gold foil without any deflection. Geiger and Marsden observed that a small number of particles were deflected at large angles. A minute fraction, estimated to be about 1 in every 8,000 to 12,000 alpha particles, was observed to recoil completely, bouncing straight back toward the source. These rare, dramatic deflections directly contradicted the predictions of the Plum Pudding Model, indicating that the internal structure of the atom was far from a uniform, soft sphere.
Proving the Existence of the Nucleus
The presence of the deflected and recoiling alpha particles forced Rutherford to conclude that the atom’s structure was fundamentally different from what was previously assumed. The only way to account for the massive deflection of the small number of alpha particles was if they were encountering a concentrated, powerful force within the atom.
Rutherford theorized that the atom must contain a tiny, dense, positively charged center, which he named the nucleus. Since the alpha particles were themselves positively charged, the immense repulsive force needed to reverse their direction could only come from a high concentration of positive charge and mass in a small volume. The nucleus, although occupying only a minute fraction of the atom’s total volume, must contain nearly all of the atom’s mass.
This conclusion established the foundation for the Rutherford model of the atom, where a small, dense nucleus is orbited by electrons, with the majority of the atom being empty space. The fact that most alpha particles passed straight through showed that the nucleus was extraordinarily small compared to the overall size of the atom. The gold foil experiment therefore proved that the atom’s positive charge and mass are not uniformly distributed but are instead localized in a minuscule, highly concentrated core.