Frederick Soddy, Radioactivity & Soddy Circles

September 2, 1877 - birthday of Frederick Soddy, an English radiochemist who explained, with Ernest Rutherford, that radioactivity is due to the transmutation of elements, now known to involve nuclear reactions. He also proved the existence of isotopes of certain radioactive elements.

In 1900 he became a demonstrator in chemistry at McGill University in Montreal, where he worked with Ernest Rutherford on radioactivity. He and Rutherford realized that the anomalous behaviour of radioactive elements was because they decayed into other elements. This decay also produced alpha, beta, and gamma radiation.

In 1903, with Sir William Ramsay, Soddy showed that the decay of radium produced helium gas. In the experiment a sample of radium was enclosed in a thin-walled glass envelope sited within an evacuated glass bulb. After leaving the experiment running for a long period of time, a spectral analysis of the contents of the bulb revealed the presence of helium. Later, Rutherford and Thomas Royds showed that the helium was first formed as positively charged nuclei of helium (He 2+) which were identical to alpha particles, which could pass through the thin glass wall but were contained within the surrounding glass envelope.

Soddy and Ada Hitchins (Soddy's research assistant) showed that uranium decays to radium.  They also showed that a radioactive element may have more than one atomic mass though the chemical properties are identical. Soddy named this concept isotope meaning 'same place'. The word 'isotope' was initially suggested to him by Margaret Todd. Later, J. J. Thomson showed that non-radioactive elements can also have multiple isotopes.

Soddy also showed that an atom moves lower in atomic number by two places on alpha emission, higher by one place on beta emission. This was discovered at about the same time by Kazimierz Fajans, and is known as the radioactive displacement law of Fajans and Soddy, a fundamental step toward understanding the relationships among families of radioactive elements. Soddy published The Interpretation of Radium (1909) and Atomic Transmutation (1953).

In 1918 he announced discovery of a stable isotope of Protactinium, working with John Arnold Cranston. This slightly post-dated its discovery by German counterparts; however, it is said their discovery was actually made in 1915 but its announcement was delayed due to Cranston's notes being locked away whilst on active service in the First World War.

Soddy received the 1921 Nobel Prize in chemistry for his research in radioactive decay and particularly for his formulation of the theory of isotopes.

He rediscovered the Descartes' theorem in 1936 and published it as a poem, "The Kiss Precise", quoted at Problem of Apollonius. The kissing circles in this problem are sometimes known as Soddy circles.

Descartes' theorem states that for every four kissing, or mutually tangent, circles, the radii of the circles satisfy a certain quadratic equation. By solving this equation, one can construct a fourth circle tangent to three given, mutually tangent circles. The theorem is named after René Descartes, who stated it in 1643.

Source - Wikipedia