The Gold Foil Experiment, conducted by Ernest Rutherford and his team, significantly changed our understanding of atomic structure. This experiment provided the first evidence that atoms are not indivisible, uniform spheres but possess a complex internal arrangement. Its findings established the nuclear model of the atom, which is central to modern atomic theory.
Before the Experiment: The Plum Pudding Model
Before Rutherford’s work, the widely accepted atomic model was J.J. Thomson’s “plum pudding” model, proposed in 1904. This model envisioned the atom as a sphere of uniformly distributed positive charge, with negatively charged electrons embedded within it, much like plums in a pudding. The positive charge was spread out, ensuring the atom’s electrical neutrality. This model predicted that alpha particles fired at a thin material would pass through with only minor deflections.
The Experiment Setup
Rutherford’s associates, Hans Geiger and Ernest Marsden, performed the experiment, directing a beam of alpha particles towards a very thin sheet of gold foil. A fluorescent screen encircled the gold foil, detecting the alpha particles as they struck it and produced tiny flashes of light. Based on the plum pudding model, the expectation was that most alpha particles would pass straight through the foil with little to no deviation.
The Unexpected Observations
The actual observations, however, presented a contradiction to these predictions. Most alpha particles passed directly through the gold foil without any deflection. A small fraction of the particles were deflected at various angles. A very tiny fraction were scattered back towards the source, as if they had hit something dense. This unexpected outcome was likened by Rutherford to firing a cannonball at tissue paper and having it bounce back.
Rutherford’s Conclusions and the Nuclear Model
These results challenged the “plum pudding” model, as the observed large-angle deflections and bounce-backs could not be explained by a uniformly distributed positive charge. Rutherford concluded that the atom’s positive charge and nearly all of its mass must be concentrated in an exceedingly small, dense central region, which he named the nucleus. The observation that most alpha particles passed through indicated that the atom is primarily empty space, with electrons orbiting this tiny, positively charged nucleus. Rutherford’s calculations suggested that the radius of this nucleus was at least 10,000 times smaller than the atom’s overall radius, leading to the nuclear model of the atom.