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4.2 Rutherford and Atomic Structure

It was 1911, and the experimental physicist Ernest Rutherford was using the newly discovered phenomenon of radioactivity to probe the structure of matter. He placed a radioactive source to send a stream of tiny particles toward a thin sheet of gold foil. Then he used a Geiger counter to detect the particles and see what happened to them as they hit the foil. Rutherford was doing a pioneering experiment to measure the structure of the atom. The ancient Greeks had come up with the idea of atoms — tiny, indivisible units of matter far too small for the eye to see. Rutherford was using a new tool to probe this invisible world.


Ernest Rutherford, 1910. Click here for original source URL

As Rutherford studied radioactivity, he was puzzled. Previous ideas of atoms led Rutherford to think that atoms were packed tightly together like ball bearings in a jar. He expected that most of the radioactive projectiles would plough into the foil and deposit their energy there. Yet when he placed the Geiger counter directly behind the foil he found that most of the particles passed through undeviated or deflected by only small angles. The particles acted as if the foil was not there! Rutherford was forced to conclude that the gold atoms were made of mostly empty space.

The Rutherford model of an atom. Click here for original source URL.

In Rutherford’s next experiment, he placed the Geiger counter on the same side of the foil as the radioactive source. Expecting to see nothing, he was amazed when a small fraction of the incoming particles — about 1 in 1000 — bounced right back in the direction of motion. At the time Rutherford said "It was quite the most incredible event that ever happened to me in my life. It was almost as incredible as if you had fired a 15-inch shell at a piece of tissue paper and it came back and hit you." This indicated that most of the mass of the atom was contained in a small dense core or nucleus. Once in a while an incoming particle would hit the nucleus and recoil back in the direction it came. He had gained new insight into the structure of the atom.

The early 20th century was the start of a golden age in atomic physics. Rutherford and his colleagues at the University of Manchester did many elegant experiments to probe atoms. This was in the days before enormous atom-smashers. Their apparatus was built using ingenuity and the kind of components you might find in a hardware store. Rutherford was proud of his ability to do great experiments with modest equipment; he once proclaimed he could do research even at the North Pole. Rutherford won the Nobel Prize in Physics for his work and became the Director of the Cavendish Laboratory at the University of Cambridge. He ended his life as Lord Rutherford.

Rutherford was born to a poor family in New Zealand, relying on scholarships to get to college and travel overseas. He was a big bear of a man with a booming voice and an intense manner. He would sweep through the Cavendish Laboratory every evening to send people home — not to relax and spend time with their families but to think more deeply about their experiments! Rutherford was undoubtedly a tough boss but his enthusiasm for new ideas and discoveries in physics meant that students flocked to work with him. He like clear thinking and simple explanations and once said that a theory was no good unless it could be explained to a bartender.