William Ramsey suggest placing argon in separate group between chlorine and potassium (7h 22min, jan 1, 1898 y – 9h 12min, dec 31, 1898 y)
Chemically inert- did not fit with other elements on the table because had no leftover valence electrons.
Ramsey correctly predicted discovery of Neon, and it's properties.
Added to timeline:
History of the Periodic Table