Ernest Rutherford’s discovery of the proton as part of the nuclear structure of the atom contributed to a new age of nuclear physics and quantum mechanics.

Until the end of the nineteenth century, atoms were thought to be shaped like hard balls, often visualized as minute billiard balls. This model of the atom had satisfied the laws of physics up to that time, and it worked particularly well with the behavior of gases, where the gas atoms bounced off one another. However, this model of the atom failed to explain certain new phenomena observed at the beginning of the twentieth century. Philipp Lenard had observed cathode rays penetrating thin layers of material as well as the photoelectric effect, where light falling on certain metals produced electricity. By 1903, Lenard suggested that the atom contained more open space than provided by the earlier billiard-ball model or the revised “plum pudding” model, in which the atom was a positive sphere embedded by electrons to make it electrically neutral. Experimental evidence of a new model of the atom had to wait for the work of Ernest Rutherford. Atoms;structure
Protons;discovery
[kw]Rutherford Discovers the Proton (1914)
[kw]Proton, Rutherford Discovers the (1914)
Atoms;structure
Protons;discovery
[g]England;1914: Rutherford Discovers the Proton[03490]
[c]Science and technology;1914: Rutherford Discovers the Proton[03490]
[c]Physics;1914: Rutherford Discovers the Proton[03490]
Rutherford, Ernest
[p]Rutherford, Ernest;discovery of the proton
Bohr, Niels
Broglie, Louis de
Geiger, Hans
Lenard, Philipp
Pauli, Wolfgang
Schrödinger, Erwin
Soddy, Frederick

Rutherford, a brilliant student, was awarded a scholarship, and he chose to work at Cavendish Laboratory, Cavendish Laboratory where Sir Joseph John Thomson was the leading authority in electromagnetism. Rutherford initially worked on problems associated with X rays and then became interested in radioactivity. In 1898, he was appointed professor of physics at McGill University in Montreal, where he continued his work on radioactivity. In 1902, assisted by Frederick Soddy, Rutherford developed the first major breakthrough in nuclear physics: an explanation of radioactive disintegration. Rutherford and Soddy suggested that radioactive decay involved the transmutation of one element to another over a specific half-life period. The half-life of a radioactive element is that period when half of a given amount has changed to a sister element. A half-life can be short or, as in the case of uranium and radium, can cover thousands of years. From this beginning, Rutherford produced a number of important papers dealing with the nature of radiation; in 1908, his work culminated in his being awarded the Nobel Prize in Chemistry. Nobel Prize recipients;Ernest Rutherford[Rutherford]

The process of radioactive decay Radioactive decay produced alpha and beta particles, and Rutherford began to concentrate on the characteristics of the more massive alpha particle. By that time, Rutherford was convinced that alpha particles Alpha particles were essentially similar to helium atoms, and he began to use this heavy particle to explore the nucleus of the atom. In 1907, Rutherford began a series of experiments at the University of Manchester on the penetration abilities of the alpha particle. These experiments led him to his greatest discovery: the nuclear structure of the atom. Rutherford first attempted to measure the number of alpha particles given off by one gram of radium. One of Rutherford’s associates, Hans Geiger, developed an instrument (today known as the Geiger counter) that could be used for this purpose.

Ernest Rutherford.

(The Nobel Foundation)

A second series of experiments was set up to measure the scattering of the alpha particles that involved the scintillation method. When an alpha particle strikes a screen of zinc sulfide, the point of contact glows in the dark. The most successful experiment resulted from the effort of a graduate student, Ernest Marsden, Marsden, Ernest who prepared an experiment to measure the scattering effect of alpha particles directly by bouncing them from foils of different materials. In 1909, this experiment produced dramatic results. Alpha particles were scattered through a number of angles, including some that reversed their directions. Rutherford compared this to firing a cannon shot at a piece of tissue paper and having the cannon ball come back at him. Given that alpha particles travel at more than 16,000 kilometers (9,942 miles) per second, this was a remarkable effect.

By 1913, Rutherford worked out the mathematical probability of the various scattering angles and hypothesized the size of the positive charge at the center of the atom. His figures indicated an atom that consisted largely of empty space, with an extremely dense nucleus at the center containing a number of positively charged protons equal to the negative charges of the electron, rendering the atom electrically neutral. In a series of papers published in 1914, Rutherford described his hypothesis of a new atomic model that included the proton as the nucleus of the atom, although he did not name the proton until later.

The scientific community paid little attention to this new model of the atom. It was too great a departure from the traditionally accepted model, and, at the time, several insurmountable problems were associated with the new concept. For one, if the electrons orbited the nucleus, they would emit radiation, which in turn would cause a loss of energy, and eventually the electrons would fall into the center of the atom. Although the theoretical foundation of nuclear physics was to come from elsewhere, Rutherford continued his demonstrations of the validity of his atomic model. In 1917, one experiment involved shooting alpha particles into a container of hydrogen. The scintillations produced by the collisions indicated that hydrogen protons were being hit by alpha particles.

The Rutherford model of the atom Rutherford model of the atom gained prominence through the work of Niels Bohr. Bohr saved Rutherford’s new model by proposing a quantum theory, in which the energy of the spinning electron emitted energy only in specific quanta. This meant that during stable orbits of the electron, there was no emission of radiation, but when the electron jumped from one orbit to another through increasing or decreasing energy of the atom, there would be emissions of radiation. Furthermore, the quantum numbers for the electrons were whole numbers. This idea was supported by the characteristic spectral line of the hydrogen atom, but it failed to account for the fine detail of the hydrogen spectrum. Although this model of the atom was later referred to as the “Bohr atom,” credit must be given to Rutherford for providing the basic structure of the atom.

The contributions of Rutherford, Bohr, and their predecessors heralded the age of nuclear physics and quantum mechanics. The 1920’s provided one of the most fertile, creative, and imaginative periods in the history of physics. As various physicists modified and changed the Bohr atom, Bohr model of the atom the model of the atom that had begun as a miniature version of the solar system now could be described only in mathematical terms, without a visual counterpart. In 1925, Wolfgang Pauli formulated the exclusion principle, Exclusion principle which stated that no two electrons on a specific atom have the same quantum number. By allowing for only four quantum numbers, it was possible to arrange all the elements of the periodic table into shells and subshells around the nucleus. This classification of elements provided chemists with a useful model for the purposes of interpreting chemical reactions. For physicists, however, this model did not account for problems in the spectral lines of elements.

Two major contributions in 1925 drastically altered the form and methodology of nuclear physics. First, Werner Heisenberg Heisenberg, Werner decided to abandon all attempts to create visual representations of atoms and proposed only mathematical relationships between the electrons and the spectral lines of the elements. Second, Louis de Broglie argued that matter and radiation both possessed properties of wave motion as well as particles. In 1926, Erwin Schrödinger brought these developments together and placed the electron into a cloudlike shell, where it was no longer possible to detect the precise location of the particle, but only its wave motion.

Meanwhile, exploration of the nucleus of the atom continued. By 1920, Rutherford not only had discovered the proton but also had proposed the neutron. The proton provided a positive charge and half the atomic weight of the element; the neutron held no electrical charge but provided the other half of the atomic weight. In 1932, James Chadwick, a student of Rutherford, supplied experimental evidence for the existence of the neutron. Both Rutherford and Chadwick had pioneered the method of bombarding the atomic nucleus with particles of higher and higher energy. These experiments marked the beginning of high-energy physics and the discovery of about one hundred subatomic particles. Nuclear physics had finally arrived as a field of study with military and commercial consequences. Atoms;structure
Protons;discovery

• Broglie, Louis de. The Revolution in Physics: A Non-mathematical Survey of Quanta. Translated by Ralph W. Niemeyer. New York: Noonday Press, 1953. A highly recommended text for those seeking nontechnical information on quantum mechanics. Attempts to provide a popular explanation of the rapidly changing world of physics and makes the difficult subject of quantum mechanics accessible to the general reader.
• 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.
• Crowther, J. G. The Cavendish Laboratory, 1874-1974. New York: Science History Publications, 1974. Describes the history of the laboratory, focusing on Joseph John Thomson, who provided the foundations and research directions of this institution for many years. Eight chapters provide information on Thomson’s successor, Ernest Rutherford.
• Einstein, Albert, and Leopold Infeld. The Evolution of Physics: The Growth of Ideas from Early Concepts to Relativity and Quanta. 1938. Reprint. New York: Simon & Schuster, 1966. Probably one of the most accessible single-volume histories on the development of modern physics available to the general reader. Uses virtually no technical terms, and no mathematics background is required. The sections on the decline of the mechanical view and on quanta are highly recommended.
• Jammer, Max. The Conceptual Development of Quantum Mechanics. 2d ed. Los Angeles: Tomash, 1989. Traces both the physics and the conceptual framework of quantum theory. Sections dealing with the formative development of quantum theory are moderately accessible for the general reader.
• Piel, Gerard. The Age of Science: What Scientists Learned in the Twentieth Century. New York: Basic Books, 2001. An overview of the scientific achievements of the twentieth century. Chapter 3 discusses Rutherford’s work. Includes many illustrations and index.
• Segrè, Emilio. From X-Rays to Quarks: Modern Physicists and Their Discoveries. San Francisco: W. H. Freeman, 1980. Segrè was one of the few physicists who both participated directly in nuclear physics (for which he received a Nobel Prize) and wrote a number of popular accounts on the history of physics. The earlier sections of this volume cover the discoveries and theories of those who produced a coherent picture of the atom.
• Trefil, James S. From Atoms to Quarks: An Introduction to the Strange World of Particle Physics. New York: Charles Scribner’s Sons, 1980. An excellent introduction to the world of subatomic particles. Chapter 4 on antimatter, chapter 6 on accelerators, and chapter 7 on the discovery of particles are highly recommended.

Becquerel Wins the Nobel Prize for Discovering Natural Radioactivity

Boltwood Uses Radioactivity to Determine Ages of Rocks

Thomson Confirms the Possibility of Isotopes

Bohr Uses Quantum Theory to Identify Atomic Structure

Rutherford Describes the Atomic Nucleus

Discovery of the Compton Effect

Gamow Explains Radioactive Alpha Decay with Quantum Tunneling

Chadwick Discovers the Neutron