A Level Secondary Science

Chemistry in 1913 part 3 – Isotopes

In 1912, Dr Margaret Todd suggested the word isotopes for atoms of the same element that had different atomic masses. Dr Todd was not a chemist or physicist, so why did she come to suggest this important scientific term? The answer involves Frederick Soddy and Ernest Rutherford, who played an important role in the first two articles about the chemistry of 1913.

In 1900, Rutherford was at McGill University at Montreal in Canada. He was investigating radioactivity, the new science begun by Henri Becquerel and Marie and Pierre Curie in Paris. Rutherford discovered that radioactive materials gave out three types of radiation – alpha, beta and gamma rays. Rutherford suspected that when atoms of a radioactive element gave out radiation they became atoms of a different element, a process called transmutation. This idea was like blasphemy to scientists at the start of the twentieth century because it sounded very much like the claim of ancient alchemists to be able to turn lead into gold. Dalton’s Atomic Theory, which by 1900 almost every scientist had accepted, states that atoms are indestructible; they cannot suddenly change from one into another. Rutherford was a physicist so he needed a chemist to investigate his controversial idea.

Frederick Soddy was born in Eastbourne, Sussex in 1877. He studied at University College Aberystwyth, in Wales and then at Oxford University where he completed a degree in chemistry. He did two more years of research at Oxford before moving to Montreal to join Rutherford’s team. Soddy was an extremely talented chemist able to separate and identify the smallest amounts of elements. At first he couldn’t accept the transmutation idea but experiments soon convinced him that Rutherford was correct. Together they investigated the chain of disintegrations as atoms of uranium, thorium, radium and other radioactive elements turned into other elements. Soddy was able to separate compounds of the elements that were formed, examine their spectra and measure their atomic weights (what we now call atomic mass). It was clear that atoms of one element were turning into atoms of another.

Soddy only worked with Rutherford for two years before returning to England to work with William Ramsay at University College, London. They proved that helium was formed when radioactive elements gave out alpha rays. This was a clear sign that Dalton’s Atomic Theory needed modification. In 1904 Soddy moved again, to the University of Glasgow, but he continued his investigations into the products of radioactive decay.

Over the next nine years, Soddy and others detected many apparently new elements formed as one radioactive atom decayed into another and gave off radiation. Some were given new names such as ionium, while others such as mesothorium and uranium X hinted at the elements they were derived from. Despite only having tiny amounts of these new materials to test, measurements showed that they had different atomic masses. That meant that they were separate elements.

Soddy noticed, however, that many of these new elements had similar chemical properties and were separated from other elements by the same chemical methods. He found that some of the new elements could not be separated by chemical means because they had identical chemical properties. Nevertheless they differed in their atomic masses, the rate they decayed and the type of radiation given out. Soddy came to the conclusion that another of Dalton’s Atomic Theory statements had to be changed, that is, that the atoms of elements are not identical. Atoms of an element had the same chemical properties but may differ in atomic mass. At first Soddy thought this only applied to radioactive elements at the bottom of the Periodic Table, but it was later found to apply to other elements which are not normally found to be radioactive.

Soddy and others could now start to clear up the muddle in the Periodic Table caused by the discovery of all these new atoms which were in fact not new elements. In papers published just two weeks apart Kasimir Fajans and Soddy came up with the same set of rules. When an atom gives out an alpha particle the product is an element two places to the left in the Periodic Table; one that gives out a beta particle forms a product one place to the right in the Table. As Fajans’ paper was first the rules have tended to be known as Fajans’ rules.

While preparing his paper Soddy met up with his cousin, Margaret Todd, one of the first women doctors to qualify from the Edinburgh School of Medicine for Women. As they discussed Soddy’s work she suggested the term isotope, meaning “at the same place” in Greek, for these atoms which occupied the same place in the Periodic Table despite their different atomic masses. Soddy introduced the term in his 1913 paper.

Soddy was also able to explain the existence of isotopes of elements. Moseley’s work had shown that it was the atomic number that determined the properties of an element and its place in the Periodic Table. Atomic mass was now of secondary importance. Isotopes were atoms with the same atomic number but different atomic mass. The Rutherford-Bohr atom with a heavy nucleus surrounded by shells of electrons showed that the chemical properties of an elements were determined by the arrangement of the electrons while radioactivity was a property of the nucleus. As Soddy put it, isotopes are atoms “with identical outsides but different insides”.

Soddy was awarded the 1921 Nobel Prize for Chemistry for his work on isotopes. In 1914 he had moved to Aberdeen University and in 1919 he became Lee’s Professor of Chemistry at Oxford University but he had given up doing chemical research. He had become more interested in economics and published many ideas for improving the economy of nations. Unlike Rutherford he could foresee a time when the energy contained in the nuclei of atoms would become an important energy resource. He thought Atomic or Nuclear energy must replace the limited stocks of fossil fuels.

Activities

1 Find out what is stated in Dalton’s Atomic Theory of 1805. Which of the statements have been shown to be untrue by Rutherford, Soddy and others?

2 Use Fajans’/Soddy’s rules to state the name of the element formed when
(a) an atom of uranium gives out an alpha particle,
(b) an atom of radium gives out a beta particle.

3 Why did Rutherford need a chemist to help him with his work in Montreal?

4 What reasons did Soddy have for thinking that atoms with different atomic masses were in fact the same element?

5 Why do you think it took over ten years for Soddy’s ideas about isotopes to be proved?

6 Why is it important to get a scientific paper published as quickly as possible? (Fajans’ paper was published late in December 1912 and Soddy’s in early January 1913. Soddy had not seen Fajans’ paper before his was published)

7 Investigate some of the other people involved in the discovery of radioactivity and isotopes e.g. William Ramsay, Otto Hahn, Lise Meitner, Bertram Boltwood, and Kasimir Fajans (there are many others involved in the story).

8 Suggest reasons why “isotope” is a good scientific term.

9 Why was 1913 an important year for chemistry?

Bibliography

Biography of Frederick Soddy
http://www.nobelprize.org/nobel_prizes/chemistry/laureates/1921/soddy-bio.html
Soddy’s Nobel Prize lecture given in 1922

http://www.nobelprize.org/nobel_prizes/chemistry/laureates/1921/soddy-lecture.pdf

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