Today is the birthday of Frederick Soddy, an English chemist born in 1877 who pioneered the study of isotopes, elements with the same number of protons but a different number of neutrons. While researching the disintegration of radioactive elements with Ernest Rutherford, Soddy in 1910 observed that elements of different atomic weights sometimes had the same chemical properties. He theorized that this could occur not only in radioactive elements, but in stable ones as well.
Knowing that uranium and thorium decay into two lead "isotopes"--which can be distinguished by their different physical properties--Soddy later measured the atomic weights of this stable element in ores rich in uranium and thorium, and found that they were 206.08 and 207.69, respectively. In 1920, he speculated that by measuring the amount of a radioactive isotope and its daughter products in rocks, geologists would be able to tell their age--a technique that has become widely used. In 1921, Soddy received the Nobel Prize for Chemistry. He died in 1956.
[Source: Britannica Online]