discovery of isotopes

B3. ISOTOPES & RELATIVE ISOTOPIC MASSES

Each element, by definition, can have only one atomic number. Most elements, however, have atoms possessing more than one mass number. Indeed, for the 118 or so elements, several thousand different nuclides have been reported in the scientific literature although some of these are likely to be of dubious existence, i.e., very short-lived and unstable species.

See https://applets.kcvs.ca/AtomicWeightCalculator/atomicWeight.html

3.1 DISCOVERY OF ISOTOPES

Read out about the Discovery of Isotopes and the contributions of F.W. Aston, J.J. Thomson and Frederick Soddy along with that of Margaret Todd. In 1913 she was to suggest the term ‘isotope’ (Greek for ‘at the same place’) to her distant relation Frederick Soddy. It suited perfectly:
Soddy, educated at Eastbourne College and Merton College, Oxford, went on to win the
Nobel Prize for Chemistry in 1921, the first English-born Chemist to receive this honour.

A good starting point might be the well-researched and written article

Making isotopes matter: Francis Aston and the mass-spectrograph

Discovery of isotopes - Aston, Thomson & Soddy

His most important research into the chemistry of radioactive elements was carried out while a lecturer in Physical Chemistry and Radioactivity during his ten years at the University of Glasgow (1904-1914). He took up the Chair of Chemistry at the University of Aberdeen in 1914 and in 1919 became Dr Lee’s Professor of Physical and Inorganic Chemistry at the University of Oxford.

Mass number can be found using a mass spectrograph, an instrument invented by F.W. Aston - a student of J.J. Thomson - in 1918. Surprisingly, results from early mass spectra showed that the atoms of pure elements did not always possess the same mass, e.g., a naturally occurring sample of neon appeared to contain atoms with two distinct nuclear structures of slightly differing masses, 20-Ne & 22-Ne, while chlorine showed up as 35-Cl & 37-Cl.

Frederick Soddy
(1877 - 1956)

Francis W. Aston
(1877 - 1945)

Francis W. Aston was awarded the 1922 Nobel Prize in Chemistry “for his discovery, by means of the mass spectrograph, of isotopes, in a large number of non-radioactive elements, and for his enunciation of the whole-number rule”.

He was born in Harbrone, now part of Birmingham, and educated at Harborne Vicarage School and Malvern College. His university studies began at Mason Science College (later part of University of Birmingham) but in 1910, at the invitation of J.J. Thomson, he moved to the Cavendish Laboratory, Cambridge.