Which element was first discovered by spectroscopy in 1860?

The element first discovered by spectroscopy in 1860 was caesium. Robert Bunsen and Gustav Kirchhoff identified it through the blue lines in the emission spectrum of mineral water from Dürkheim, Germany.

The discovery demonstrated the power of flame spectroscopy, a technique developed only shortly beforehand. When a substance is heated, its atoms emit characteristic wavelengths of light; these spectral lines can reveal elements even when they are present in tiny amounts. The name caesium comes from the Latin caesius, meaning “sky blue.”

Caesium was not isolated as a pure metal until 1882, when Carl Setterberg obtained it by electrolysis of caesium cyanide. A common mix-up is rubidium, which Bunsen and Kirchhoff discovered spectroscopically soon afterward, in 1861. Caesium later became especially important in atomic clocks: the SI definition of the second uses a microwave transition in caesium-133.