Boltwood, Bertram (1870–1927)

US radiochemist who pioneered the use of radioactive elements as tools for dating rocks and determined the age of the earth to be at least 2.2 billion years old. He discovered ionium and also made the first observations of the phenomenon of isotopy.

By studying abundances of radioactive elements in ores, Boltwood deduced that the radium present in an ore was the product of the breakdown of uranium in the ore and that uranium ultimately would decay to lead. In 1907, he demonstrated that by knowing the rate at which uranium decays (its half-life) he could calculate the age of a mineral by measuring the relative proportions of its uranium and lead. Boltwood dated rocks from several localities using his uranium–lead technique, obtaining ages between 410 million to 2.2 billion years old. His efforts showed that the earth was significantly older than previously thought.

Boltwood was born in Amherst, Massachusetts, and studied chemistry at Yale from 1889 to 1992. He went on to study at Munich and Leipzig before returning to Yale as a professor 1897–1900. In 1900, he left Yale to work with geologist J H Platt. It was during this time that Ernest Rutherford and Frederick Soddy discovered that radioactive elements, such as thorium and uranium, turn into other elements by radioactive decay.

Boltwood returned to Yale in 1907 as a professor of radiochemistry. He went on to discover ionium. In his efforts to separate ionium from thorium he made the first observations of isotopes, since ionium is not a separate element, but actually an isotope of thorium.