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Physical Chemistry

Celebrating The Isotope

Frederick Soddy proposed that elements could have several atomic weights 100 years ago this month

by George B. Kauffman
December 2, 2013 | A version of this story appeared in Volume 91, Issue 48

Credit: University of Glasgow Archive Services, GB0248 UP1/503/1
Soddy in his lab at the University of Glasgow.
Photo of Frederick Soddy in his lab at the University of Glasgow.
Credit: University of Glasgow Archive Services, GB0248 UP1/503/1
Soddy in his lab at the University of Glasgow.

In a letter to the editor published in the Dec. 4, 1913, issue of Nature, English radiochemist Frederick Soddy proposed the isotope concept—that elements could have more than one atomic weight. The idea led to his 1921 Nobel Prize in Chemistry. The notion of isotopes and the rest of Soddy’s lifetime body of work on radioactivity, the group displacement laws, the social responsibility of scientists, and the environment have had a profound effect on science and society.

Soddy was born in Eastbourne, En­gland, on Sept. 2, 1877. His mother died when he was 18 months old, and he was raised by his half-sister in the Calvinist tradition, which imbued him with an exceptional sense of social independence. But he also acquired a critical attitude toward religion, which he later extended to social institutions in general. A precocious youth, he studied at Eastbourne College; Aberystwyth College of Wales; and Merton College, University of Oxford. Soddy conducted independent research at Oxford from 1898 to 1900.

In 1900, Soddy became a demonstrator in chemistry at McGill University in Montreal. At McGill, he worked on radioactivity with New Zealand-born British physicist Sir Ernest Rutherford, the father of nuclear physics and a future (1908) Nobel Laureate in chemistry.

At the time, radioactivity was poorly understood, but Soddy and Rutherford realized that the phenomenon was caused by the decay of elements into other elements with simultaneous emission of α-, β-, and γ-radiation. Soddy used chemical methods to identify the products of the decay. During an 18-month period between 1900 and 1902, Rutherford and Soddy published nine historic articles proposing that the atoms were breaking apart and forming new kinds of matter. This was the dream of the ancient alchemists, and Soddy called the process “transmutation,” a term he borrowed from alchemy.

Rutherford and Soddy advanced a disintegration theory of radioactivity explaining how this process occurred. They were the first to calculate the tremendous amount of energy that could be evolved during radioactive transmutations. Their theory was so complete that it formed the basis for all later studies. Because Soddy was erroneously considered a junior partner instead of a younger colleague, Rutherford garnered most of the credit for their work. Therefore, Soddy was not nominated for the 1908 Nobel Prize in Chemistry that resulted from that work.

The duo showed the existence of two radioactive series, beginning with uranium-92 and thorium-90, with lead-82 being the final product of both series. They predicted that helium should be the decay product of radium-88.

In 1903, Soddy left Canada to work at University College London with Scottish chemist Sir William Ramsay. Soddy and Ramsay showed conclusively by spectroscopic analysis that helium is produced by expulsion of α-particles during the disintegration of radium—the first experimental proof of the natural transmutation of elements proposed earlier by Soddy and Rutherford. α-Particles are doubly charged particles consisting of two protons and two neutrons, identical with the helium nucleus, He2+.

From 1904 to 1914, Soddy served as a lecturer at Glasgow University, Scotland. In 1908, he married Winifred Beilby, whose father, Sir George T. Beilby, was a distinguished chemist in his own right and general manager of the Cassel Cyanamid Co. of Glasgow. The couple had no children; Soddy thought that his work with radioactivity had rendered him sterile.

At Glasgow, Soddy evolved his “group displacement law,” which stated that emission of an α-particle from a radioactive element causes that element to move two places to the left in the periodic table and that emission of a β-particle (an electron) causes that element to move one place to the right in the table. When an element emits an α-particle and then emits two β-particles, it returns to its original place in the periodic table. It is the same element with all the same chemical properties but differs in mass.

In a short letter to the editor of Nature in 1913, Soddy proposed the term “isotope” to designate chemically identical elements with different atomic weights—in modern terms, elements with the same atomic numbers but different mass numbers. Isotopes of an element occupy the same place in the periodic table. (In Greek, ίσοs means same and τόποs means place.) According to Soddy’s recollection, he devised the term when he “got tired of writing ‘elements chemically identical and non-separable by chemical methods.’ ”

According to Soddy’s student Alexander Fleck, “We owe the word ‘isotopes’ to Dr. Margaret Todd, a medically qualified Glasgow friend of Mr. and Mrs. Soddy who made her name as a writer using the pen name of Graham Travers. This word was coined at a dinner party.”

Within a single month in 1914, Soddy and Henry Hyman and three other groups—Maurice Curie, Marie Curie’s nephew; Otto Hönigschmid and Stephanie Horovitz; and Theodore William Richards and Max Ernst Lembert—proved the isotope concept experimentally by showing that lead produced by disintegration of radium differed in atomic weight from ordinary natural lead.

Soddy received the 1921 Nobel Prize in Chemistry “for his contributions to our knowledge of the chemistry of radioactive substances, and his investigations into the origin of isotopes.” In his Nobel address he said of isotopes, “Put colloquially, their atoms have identical outsides but different insides.” He had recognized that, although his concept of isotopes originated from studies of radioactive elements, they would likely be found to exist among nonradioactive elements, a prediction supported by the awarding of the 1922 Nobel Prize in Chemistry to Englishman Francis William Aston for the discovery of such isotopes.

After a stint at Aberdeen University, in Scotland, Soddy became Lee’s Professor of Chemistry at Oxford University, a position he held from 1919 to 1936. His interest shifted to the social implications of science, and he became a controversial participant in debates about a range of issues in economics, business, finance, sociology, politics, the environment, and other areas. Soddy was profoundly disturbed by World War I and enraged by the death of the brilliant young English physicist Henry Gwyn Jeffreys Moseley, whose 1913 concept of atomic numbers revolutionized the periodic table.

Soddy was driven by his perception of the need to work for the improvement of the lot of humankind and his feeling that efforts must be made to direct the fruits of science to beneficent uses to achieve this goal. He wrote: “It is modern science which has made the modern world great, with a greatness that the illustrious epochs of history cannot match.” His foreword to Sir Daniel Hall’s 1935 book, “The Frustration of Science,” is a classic call for responsibility in science.

Soddy realized earlier than most the theoretical possibility of nuclear energy, which he thought might be powering the stars. A true visionary, in his 1926 book “Wealth, Virtual Wealth, and Debt,” he asked what sort of a world it would be if atomic energy ever became available. He immediately answered, “If the discovery were made tomorrow, there is not a nation that would not throw itself heart and soul into the task of applying it to war, just as they are now doing in the case of the newly developed chemical weapons of poison-gas warfare. … If [it] were to come under existing economic conditions, it would mean the reductio ad absurdum of scientific civilization, a swift annihilation instead of a lingering collapse.”

In 1936, Soddy took early retirement following the death of his wife. His declining years were marked by unhappiness. He felt, with some justification, that his later contributions were not adequately recognized. He died in Brighton, England, on Sept. 22, 1956, at the age of 79. Soddyite, a radioactive uranium mineral, and a crater on the far side of the moon are named in his honor.

Soddy bequeathed the bulk of his estate to establish the Frederick Soddy Trust, a £25,000 educational fund. The trust, dedicated to providing opportunities for groups of youths to travel and experience other societies, has assisted hundreds of groups. And through its work, his concerns with society’s problems continue to be addressed—even in this centennial year of his landmark achievement, the concept of isotopes.

George B. Kauffman, professor emeritus of chemistry at California State University, Fresno, is the recipient of numerous awards, including several from the American Chemical Society: the Pimentel Award in Chemical Education, the Award for Research at an Undergraduate Institution, and the Helen M. Free Award for Public Outreach. A 61-year ACS member, he was selected as an ACS Fellow in 2011.


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