Rutherford Describes the Atomic Nucleus

Date March 7, 1912

Ernest Rutherford discovered the nucleus of the atom during experiments with radioactive elements, and in the process he deduced the true, divisible nature of the atom.

Locale Manchester, England

Key Figures

• Ernest Rutherford (1871-1937), English physicist
• Joseph John Thomson (1856-1940), English physicist
• Hans Geiger (1882-1945), German physicist
• Ernest Marsden (1889-1970), student of Rutherford
• Niels Bohr (1885-1962), Danish physicist

Summary of Event

Until the beginning of the twentieth century, nearly all atomic theory had come straight from philosophers. The early Greeks assumed that one could divide matter no farther than into tiny particles they called atomos, meaning indivisible. In English, atomos became “atom.” Atomic theory was made more or less respectable by the British physicist and chemistRobert Boyle in the seventeenth century, but from a modern perspective it was still philosophy without hard scientific evidence.

On April 29, 1897, atomic science was transformed from philosophy to hard science. That evening, Sir Joseph John Thomson announced that he had discovered tiny subatomic particles, which he called “corpuscles,” that were much smaller than the atom. They were later renamed “electrons.” Thomson’s discovery confirmed not only that atoms existed but also that they were probably made up of even smaller particles. According to Thomson’s theory, the atom comprised a positively charged, fluidized interior of great volume and tiny, negatively charged electrons enclosed in the fluid. Thomson described the aggregate as similar to “plum pudding.”

In 1907, Ernest Rutherford accepted a position at Manchester, England. He had been experimenting with the particles that appeared to emanate from the radioactive atom. These efforts had been narrowed to a series of experiments that he hoped would finally identify these particles and their nature. Assisting him were Hans Geiger and an undergraduate student named Ernest Marsden.

Enough data from Thomson’s work had filtered in that Rutherford and his assistants knew that the electron was a piece of the atom and that it was both lighter and smaller than the whole atom. They also knew its charge was negative; whatever was left of the atom had to be much heavier and have a net positive charge. Yet the particles that were emitted by the radioactive material Rutherford was examining—called alpha particles—were much heavier than electrons but still smaller than a whole atom. The question that perplexed Rutherford was whether, like the electron, these were also a part of the atom.

The experimental apparatus that Rutherford set up was quite simple compared with the multimillion-dollar devices used by physicists a century later. It consisted of a glass tube that held an alpha particle emitter at one end. At the other end was a target of gold foil and beyond that a fluorescent screen that acted as the detector.

The theory behind Rutherford’s experiments was that the alpha particle from the radioactive element would race down the tube from its source and strike the atoms in the gold foil. If the atoms were made up of Thomson’s “plum pudding,” then as the massive alpha particle struck the electrons, they would be deflected only slightly or not at all. By measuring where the tiny blips of light struck the gold foil, Rutherford could calculate the angle of deflection and indirectly determine the mass of whatever the alpha particle had struck on its way down the tube. He reasoned that the deflections of the more massive alpha particles striking tiny electrons would be minimal, but that if, by the most bizarre of circumstances, one of these particles should encounter a series of electrons on its way through an atom, the deflection might register as much as 45 degrees.

The experiments began in 1910 with Geiger assisting Marsden, counting the almost invisible flashes of light on the fluorescent screen through a magnifying lens in a completely blackened laboratory. They immediately found an astonishing effect. One out of about eight thousand alpha particles was deflected at an angle, varying from greater than 45 degrees to 180 degrees.

It was obvious to Rutherford that plum pudding could never account for such wild deflections. He considered that perhaps the nucleus held a charge vastly greater than any hypothesized and that the alpha particle was being whipped around the interior of the atom like a comet tossed back into the deep solar system by the Sun.

The only other plausible explanation, which Rutherford eventually accepted, was that the atom contained a tiny, pinpoint nucleus that occupied only a minuscule portion of the total volume of the atom but, at the same time, itself contained nearly all the atom’s mass. The electrons, he supposed, orbited like tiny, flyweight particles at huge distances from the densely packed core. On March 7, 1912, Rutherford presented his theory at the Manchester Literary and Philosophical Society.

Rutherford had the correct idea of the nucleus. Electrons do not “orbit” in the classical sense, however; they “exist” in a quantum state, as Niels Bohr would later prove. In the process, Bohr would change the face of physics. Rutherford’s discovery would be the last major finding of classical physics. By 1913, Rutherford’s vision would be replaced by Bohr’s quantum view.

Significance

From the time of classical Greece, people had viewed matter as made up of tiny, indivisible particles. The notion was nothing more than an educated guess. It held through thousands of years not because of its inherent accuracy but because it was a useful paradigm and because of the lack of technology to prove otherwise. This idea became so firmly implanted that it became a kind of theology of reason without implicit cause. When Rutherford proved the notion of the indivisible atom wrong, he was met with immediate disbelief. At least Thomson’s atom had substance; according to Rutherford, atoms were made up mostly of space.

Bohr would soon redefine the atom in new and innovative terms. Bohr described everything equal in size or smaller than the atom in terms of quantum mechanics, which deals with the interaction of matter and radiation, atomic structure, and so forth. Rutherford explored as deeply inside the atom as one could go within the framework of knowledge current at that time. A new science had to be developed to go even deeper. Quantum mechanics would join with Albert Einstein’s work on relativity to reorder physics and redefine the nature of all matter and energy.

Bibliography

Asimov, Isaac. Understanding Physics. Vol. 3. New York: Walker, 1966. Written for the general public, this book succeeds brilliantly in not only describing physics so that it is easy to understand but also including history so that it is vital and interesting. Discusses principally the electron, proton, and neutron.

Cline, Barbara Lovett. The Questioners. New York: Thomas Y. Crowell, 1965. Biographical collection about the lives of the quantum physicists. Begins with Rutherford and a delightful, detailed discussion of what went on behind the scenes in the Rutherford team. Describes the work of the other physicists of the day, from Bohr to Einstein, and how their efforts blended into quantum physics. An exciting book for a wide audience.

Crease, Robert P., and Charles C. Mann. The Second Creation: Makers of the Revolution in Twentieth-Century Physics. Rev. ed. New Brunswick, N.J.: Rutgers University Press, 1996. Highly readable volume follows the development of physics from its nineteenth century roots to the enigmatic mysteries of the late twentieth century. Examines characters and personalities as well as the issues of physics. Discusses Rutherford’s work in detail, from his fundamental discoveries to the full implications of his collaboration with Bohr after 1912.

Cropper, William H. Great Physicists: The Life and Times of Leading Physicists from Galileo to Hawking. New York: Oxford University Press, 2001. Presents portraits of the lives and accomplishments of important physicists and shows how they influenced one another with their work. Chapter 21 is devoted to Ernest Rutherford. Includes glossary and index.

Hawking, Stephen W. A Brief History of Time. 10th anniversary ed. New York: Bantam Books, 1996. One of the most prominent physicists of our time examines the universe, from his view of creation to the late 1980’s. Discusses the whole cosmos, from the atom and its innermost parts to the far-flung reaches of space. Includes illustrations, diagrams, glossary, and index.

Pagels, Heinz R. The Cosmic Code: Quantum Physics as the Language of Nature. New York: Simon & Schuster, 1982. Embarks on a literary quest to explain some of the most profoundly difficult topics in quantum physics in a nontechnical manner for the lay reader. Offers a concise view of the evolution of atomic theory, from the work of Rutherford to the most complex grand unified theories. Illustrated.