The invention: The first device used to measure the mass of atoms, which was found to be the result of the combination of isotopes. The people behind the invention: Francis William Aston (1877-1945), an English physicist who was awarded the 1922 Nobel Prize in Chemistry Sir Joseph John Thomson (1856-1940), an English physicist William Prout (1785-1850), an English biochemist Ernest Rutherford (1871-1937), an English physicist Same Element, Different Weights Isotopes are different forms of a chemical element that act similarly in chemical or physical reactions. Isotopes differ in two ways: They possess different atomic weights and different radioactive transformations. In 1803, John Dalton proposed a new atomic theory of chemistry that claimed that chemical elements in a compound combine by weight in whole number proportions to one another. By 1815, William Prout had taken Dalton’s hypothesis one step further and claimed that the atomic weights of elements were integral (the integers are the positive and negative whole numbers and zero) multiples
of the hydrogen atom. For example, if the weight of hydrogen was 1, then the weight of carbon was 12, and that of oxygen 16. Over the next decade, several carefully controlled experiments were conducted to determine the atomic weights of a number of elements. Unfortunately, the results of these experiments did not support Prout’s hypothesis. For example, the atomic weight of chlorine was found to be 35.5. It took a theory of isotopes, developed in the early part of the twentieth century, to verify Prout’s original theory. After his discovery of the electron, Sir Joseph John Thomson, the leading physicist at the Cavendish Laboratory in Cambridge, England, devoted much of his remaining research years to determining the nature of “positive electricity.” (Since electrons are negatively charged, most electricity is negative.) While developing an instrument sensitive enough to analyze the positive electron, Thomson invited FrancisWilliam Aston to work with him at the Cavendish Laboratory. Recommended by J. H. Poynting, who had taught Aston physics at Mason College, Aston began a lifelong association at Cavendish, and Trinity College became his home. When electrons are stripped from an atom, the atom becomes positively charged. Through the use of magnetic and electrical fields, it is possible to channel the resulting positive rays into parabolic tracks. By examining photographic plates of these tracks, Thomson was able to identify the atoms of different elements. Aston’s first contribution at Cavendish was to improve the instrument used to photograph the parabolic tracks. He developed a more efficient pump to create the required vacuum and devised a camera that would provide sharper photographs. By 1912, the improved apparatus had provided proof that the individual molecules of a substance have the same mass. While working on the element neon, however, Thomson obtained two parabolas, one with a mass of 20 and the other with a mass of 22, which seemed to contradict the previous findings that molecules of any substance have the same mass. Aston was given the task of resolving this mystery. Treating Particles Like Light In 1919, Aston began to build a device called a “mass spectrograph.” The idea was to treat ionized or positive atoms like light. He reasoned that, because light can be dispersed into a rainbowlike spectrum and analyzed by means of its different colors, the same procedure could be used with atoms of an element such as neon. By creating a device that used magnetic fields to focus the stream of particles emitted by neon, he was able to create a mass spectrum and record it on a photographic plate. The heavier mass of neon (the first neon isotope) was collected on one part of a spectrum and the lighter neon (the second neon isotope) showed up on another. This mass spectrograph was a magnificent apparatus: The masses could be analyzed without reference to the velocity of the particles, which was a problem with the parabola method devised by Thomson. Neon possessed two isotopes: one with a mass of 20 and the other with a mass of 22, in a ratio of 10:1. When combined, this gave the atomic weight 20.20, which was the accepted weight of neon.Aston’s accomplishment in developing the mass spectrograph was recognized immediately by the scientific community. His was a simple device that was capable of accomplishing a large amount of research quickly. The field of isotope research, which had been opened up by Aston’s research, ultimately played an important part in other areas of physics.Impact The years following 1919 were highly charged with excitement, since month after month new isotopes were announced. Chlorine had two; bromine had isotopes of 79 and 81, which gave an almost exact atomic weight of 80; krypton had six isotopes; and xenon had even more. In addition to the discovery of nonradioactive isotopes, the “whole-number rule” for chemistry was verified: Protons were the basic building blocks for different atoms, and they occurred exclusively in whole numbers. Aston’s original mass spectrograph had an accuracy of 1 in 1,000. In 1927, he built an even more accurate instrument, which was ten times more accurate. The new apparatus was sensitive enough to measure Albert Einstein’s law of mass energy conversion during a nuclear reaction. Between 1927 and 1935, Aston reviewed all the elements that he had worked on earlier and published updated results. He also began to build a still more accurate instrument, which proved to be of great value to nuclear chemistry. The discovery of isotopes opened the way to further research in nuclear physics and completed the speculations begun by Prout during the previous century. Although radioactivity was discovered separately, isotopes played a central role in the field of nuclear physics and chain reactions.