Timeline of chemical element discoveries Modern discoveries Modern discoveries

| Z | Element_ | Observed_or_predicted | Isolated_widely_known | Observer | First_isolator | Notes_ |
|---|---|---|---|---|---|---|
| 55 | Caesium | 1860 | 1882 | R. Bunsen and R. Kirchhoff | C. Setterberg | Bunsen and Kirchhoff were the first to suggest finding new elements by spectrum analysis. They discovered caesium by its two blue emission lines in a sample of Dürkheim mineral water. The pure metal was eventually isolated in 1882 by Setterberg. |
| 37 | Rubidium | 1861 |  | R. Bunsen and G. R. Kirchhoff | R. Bunsen | Bunsen and Kirchhoff discovered it just a few months after caesium, by observing new spectral lines in the mineral lepidolite. Bunsen never obtained a pure sample of the metal, which was later obtained by Hervesy. |
| 81 | Thallium | 1861 | 1862 | W. Crookes | C.-A. Lamy | Shortly after the discovery of rubidium, Crookes found a new green line in a selenium sample; later that year, Lamy found the element to be metallic. |