Ernest Rutherford Discovers the Nucleus

Ernest Rutherford
(1871 – 1937)

On December 20, 1910, New Zealand born physicist Ernest Rutherford made his seminal gold foil experiment which led to first insight about the nature of the inner structure of the atom and to the postulation of Rutherford’s concept of the “nucleus”, his greatest contribution to physics. Most interestingly, Rutherford made his greatest discovery after receiving the Nobel Prize in chemistry in 1908.

Ernest Rutherford was born on August 30, 1871, as the fourth child of 12 born to James Rutherford, a farmer, and his wife Martha Thompson, originally from Hornchurch, Essex, England, who had emigrated emigrated to New Zealand. His first name was mistakenly spelled ‘Earnest’ when his birth was registered. At age ten Ernest received his first science book. Amongst the many suggested experiments in it one, on using the speed of sound to determine the distance to a firing cannon, gave him the knowledge to surprise his family by estimating the distance to a lighting flash. Perhaps it was also this book which inspired him to make a minature cannon out of a hat peg, a marble and blasting powder. The cannon exploded, luckily without causing injury. Martha Rutherford ensured that all her children were well prepared for school and all received good educations. In 1887 Ernest won, on his second attempt, the Marlborough Education Board scholarship to Nelson College. In 1889 he was head boy, played in the rugby team and, again on his second attempt, won a scholarships at the University of New Zealand. He graduated M.A. in 1893 with a double major in Mathematics and Physical Science, and he continued with research work at the College for a short time, receiving his Bachelor of Science degree the following year.

The same year he was awarded a Scholarship, enabling him to go to Trinity College at the University of Cambridge as a research student at the Cavendish Laboratory under J.J. Thomson. There, he briefly held the world record for the distance over which wireless waves were detected. During the investigation of radioactivity, he coined the terms alpha, beta and gamma rays. In 1897, Rutherford was awarded his B.A. Research Degree at Trinity College. In 1898, Rutherford left for Canada to take up a professorship at McGill University in Montreal, which boasted one of the best-equipped laboratories in the Western Hemisphere. There, he did the work that gained him the 1908 Nobel Prize in Chemistry, demonstrating that radioactivity was the spontaneous disintegration of atoms. Rutherford noticed that in a sample of radioactive material, it invariably took the same amount of time for half the sample to decay – its so-called “half-life” – and created a practical application for this phenomenon using this constant rate of decay as a clock. The half-life ranges from seconds to billions of years and is unique for each radioelement and thus an excellent identifying tag. This phenomenon could then be used to help determine the actual age of the Earth that turned out to be much older than most scientists at the time believed. This also led Rutherford to his famous ironic remark,

“In science there is only physics; all the rest is stamp collecting.”

After 1905, Rutherford turned his attention to the nature of alpha radiation. The deflection of alpha rays by electrical and magnetic fields demonstrated that the rays must consist of some kind of particles. Alpha rays are actually a stream of alpha particles traveling at very high rates of speed. Rutherford showed that an alpha particle is identical to a helium atom without its electrons, that is, a helium nucleus. Starting in 1909 Rutherford started experimenting with a gold foil that was bombarded wit beam of alpha particles. This was performed by Hans Geiger and Ernest Marsden under the direction of Ernest Rutherford at the Physical Laboratories of the University of Manchester. However, the achieved results surprised Rutherford. Although many of the alpha particles did pass through the gold foil as expected, many others were deflected at small angles while others were reflected back to the alpha source. They observed that a very small percentage of particles were deflected through angles much larger than 90 degrees. This meant that the atom had a concentrated center of positive charge and of relatively large mass. The alpha particles had either hit the positive center directly or passed by it close enough to be affected by its positive charge. Since many other particles passed through the gold foil, the positive center would have to be a relatively small size compared to the rest of the atom – meaning that the atom is mostly open space. This led to a new interpretation of the nature of the atomic world. Rutherford’s new description of the atom set the foundation for all future atomic models and the development of nuclear physics. Rutherford’s model was later elaborated into the Bohr model by physicist Niels Bohr in 1913. The Bohr model, in turn, was soon replaced by the Schrodinger model of the atom, as the basic atomic model used today.

At yovisto, you may enjoy a video by Dr. Brian Cox, discussing Ernest Rutherford and his seminal gold foil experiment.

References and further Reading:

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