Today is the 167th anniversary of the birth of William Ramsay, the scientist who is credited with discovering the noble gases. Google recognized the event with a doodle on 10/2/2019
When the noble gases were being discovered (Argon was the first in 1894, helium closely thereafter in 1895), Mendeleev was very doubtful they were elements. These new species did not fit within his periodic table. Additionally, they didn’t do anything like other elements (make oxides and hydrides, for example). It wasn’t until 1898, when neon, xenon and krypton were also discovered that it occurred to scientists that a new row of the periodic table had been discovered.
In 1900, the radioactive radon was discovered (although the ideas of radioactivity were still being fleshed out at that time by Marie Curie). The last noble gas to be discovered was oganesson, which happened in 2006.