Adapted from an image (public domain), via Wikimedia Commons.
Gustav Kirchhoff
Discovered rubidium (1861) and caesium (1860).
Elements
In 1860 Robert Bunsen and Gustav Kirchhoff, in Heidelberg, saw two sky-blue lines in the spectrum of mineral water from Bad Dürkheim that no known element made. They named the element caesium, from the Latin for sky blue.
In 1861 Robert Bunsen and Gustav Kirchhoff, in Heidelberg, dissolved the mineral lepidolite and found two deep red lines in its spectrum that no known element made. They named the element rubidium, from the Latin for deep red.
Kirchhoff and Bunsen showed that every element glows with its own lines of colour, and found caesium and rubidium that way. Here is the method at work on helium.
A fingerprint made of light
In 1868, astronomers watching a solar eclipse saw a yellow line at 587.6 nm that matched no element known on Earth. It was named helium, after Helios, the Greek word for the Sun, and wasn’t found on Earth until 1895.
Electrons race through the tube and crash into helium atoms, knocking the atoms’ own electrons up to higher energy levels. As those fall back, the atoms give off light only at the exact wavelengths below, so each gas glows its own colour.
The five gases’ visible lines side by side: each pattern is a fingerprint. Choose one to see it in detail below.
Click a line to see where it comes from.
388.9 nmviolet
An electron falls from the 3p level down to the 2s level. It loses 3.19 eV of energyAn electronvolt (eV) is a tiny unit of energy used for atoms: the energy one electron gains crossing 1 volt. A particle of visible light carries between about 1.7 eV (red) and 3.3 eV (violet). and gives it off as one particle of violet light. Bigger drops give bluer light.
The strongest visible lines of neutral helium from the NIST Atomic Spectra Database. Line brightness follows NIST’s relative intensities, which only compare lines of the same element. Colours are approximate, and lamp colours are as they look to the eye.
Keep exploring
Where the dates and stories on this page come from; each event on the timeline links to its own source.
- Gustav KirchhoffEncyclopaedia Britannica
- CaesiumRoyal Society of Chemistry
- RubidiumRoyal Society of Chemistry
- Gustav KirchhoffWikipedia