12.2. ALPHA PARTICLE SCATTERING AND RUTHERFORD'S NUCLEAR MODEL OF THE ATOM

At the suggestion of Ernst Rutherford, in 1911, H. Geiger and E. Marsden performed some experiments. In one of their experiments, they directed a beam of 5.5 MeV a-particles emitted from a 214Bi radioactive source at a thin metal foil made of gold.

In 1911, under Rutherford's guidance, Geiger and Marsden conducted a pivotal experiment. They aimed a beam of alpha particles, which are positively charged particles from the decay of certain radioactive materials, at a very thin gold foil. The purpose was to observe how these alpha particles interacted with the atoms in the gold foil, helping to uncover details about the atomic structure.

Imagine shooting a ping pong ball at a thin curtain. If most balls pass through without hitting anything, a few bounce back, and perhaps some hit hard and change direction. This is similar to what happened in this experiment: most alpha particles went through the foil, while only a few deflected back.

The beam was allowed to fall on a thin foil of gold of thickness 2.1 × 10–7 m. The scattered alpha-particles were observed through a rotatable detector consisting of zinc sulphide screen and a microscope.

The experiment employed a very thin layer of gold foil, just a fraction of a millimeter thick. As alpha particles impacted here, a detector, which included a screen that emitted flashes of light, was used to track and analyze the scattering of these particles. Scientists could see where and how many alpha particles were deflected from their initial paths.

Think of this as shining a laser pointer onto a thin sheet of paper. Sometimes it shines straight through, sometimes it reflects off at an angle if it hits something underneath. Similarly, the alpha particles could reflect off the gold atoms in various directions.

Many of the a-particles pass through the foil. It means that they do not suffer any collisions. Only about 0.14% of the incident a-particles scatter by more than 1°; and about 1 in 8000 deflect by more than 90°.

The outcomes of the experiments were striking. The majority of the alpha particles passed straight through the gold foil without making any contact with the atoms, indicating that atoms are mostly empty space. However, a small percentage—0.14%—deflected by small angles, and a rare 1 in 8000 bounced back at a large angle, suggesting a significant encounter.

If you throw a handful of marbles at a stack of boxes, most marbles may roll off without hitting anything—they just glide through the gaps. But a few might hit directly, bouncing back unexpectedly. This illustrates how most alpha particles behaved during the experiment.

Rutherford argued that, to deflect the a-particle backwards, it must experience a large repulsive force. This force could be provided if the greater part of the mass of the atom and its positive charge were concentrated tightly at its centre.

Based on the scattering results, Rutherford theorized that most of the atom's mass and all of its positive charge were concentrated in a small, centralized area he called the nucleus. This insight was revolutionary as it provided a clear understanding of atomic structure, differentiating it sharply from Thomson's 'plum pudding' model.

Picture the solar system where most mass (the Sun) is concentrated at the center while planets (the electrons) orbit around it. This parallel illustrates how in Rutherford's model, the dense nucleus corresponds to the Sun, and the orbiting electrons to the planets.

Rutherford's experiments suggested the size of the nucleus to be about 10–15 m to 10–14 m. The size of an atom was known to be about 10–10 m, about 10,000 to 100,000 times larger than the nucleus.

Rutherford's nuclear model provided estimates for the size of the atom and its nucleus. While the nucleus might only measure around 1/100,000th of the total size of the atom, indicating that atoms possess a vast amount of empty space, this left many questions about the nature of matter and how electrons moved around the nucleus.

If an atom were the size of a football stadium, the nucleus would be roughly the size of a marble sitting at the center of it. This helps grasp just how much room there is in an atom compared to the actual nucleus.

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