Thorium (Th) is a naturally occurring, weakly radioactive element belonging to the actinide series on the periodic table. While recognized today for its potential in nuclear energy, the story of its initial identification involved confusion and a false start. The process of classifying this heavy element spanned more than a decade in the early 19th century, when fundamental concepts of chemistry were still developing.
The First Attempt at Identification
The first attempt to identify thorium occurred around 1815, involving the Swedish chemist Jöns Jakob Berzelius. Berzelius analyzed a mineral sample from the Swedish mining town of Falun and concluded it contained a new substance. He believed he had isolated a new “earth,” or metal oxide.
He tentatively named this supposed new element “Thorium,” honoring the Norse god of thunder, Thor. However, Berzelius realized his initial analysis was incorrect after further investigation in 1824. The substance he had identified was not a new element but yttrium phosphate, a compound of an already known element.
Definitive Discovery and Naming
The definitive discovery of thorium was made by Berzelius in 1828, thirteen years after his initial mistake. The correct identification resulted from a new, unusual black mineral found near Brevik, Norway, by amateur mineralogist Morten Thrane Esmark. Esmark’s father, a professor, sent a sample of the unknown material to Berzelius for analysis.
Berzelius’s analysis of this mineral revealed that it contained an oxide entirely distinct from any other known substance. This confirmed the presence of a new element, which he successfully isolated in its oxide form (ThO₂). He then isolated the metal itself by reacting thorium chloride with potassium, a common method for obtaining elemental metals at the time.
Berzelius decided to reuse the name “Thorium,” honoring the Norse deity a second time, and the source mineral was named thorite. This definitive discovery was formally announced to the scientific community in 1829.
Thorium’s Initial Scientific Classification
Following its isolation, thorium was recognized as a dense, heavy element with a high atomic mass. In 19th-century chemical systems, it was initially placed among the heavy elements. Its chemical properties, dominated by the +4 oxidation state, positioned it near elements like titanium and zirconium, though its true nature as an actinide was not understood until the 20th century.
A major scientific revelation occurred much later, in 1898, when Gerhard Schmidt and Marie Curie independently discovered that thorium was radioactive. This property was unknown to Berzelius, as the concept of radioactivity had not yet been established. The discovery of its natural radioactivity placed it alongside uranium as a long-lived, naturally occurring radioactive element.
Before radioactivity became a primary focus, thorium found significant industrial use. In 1884, the element became the main component in the incandescent gas mantle, invented by Carl Auer von Welsbach. When heated by a flame, the thorium oxide produced a brilliant, steady white light, making it a popular source of illumination before electric lighting became widespread.