In 1902, Mendeleev placed helium and argon in which group?

In 1902, Mendeleev placed helium and argon in Group 0.

The placement reflected the gases’ striking chemical inactivity. Helium had been detected in the solar spectrum before its terrestrial isolation, while argon was identified in air by Lord Rayleigh and William Ramsay in the 1890s. Their discovery revealed a previously unrecognized family of elements that did not fit comfortably into the earlier periodic table.

Mendeleev used Group 0 for these chemically inert gases because they appeared to have no ordinary combining capacity. The zero label therefore described their apparent valence or reactivity, not an absence of atoms or mass. Later members of the family included neon, krypton, xenon and radon.

Readers often see Group 18 in modern periodic tables and assume Group 0 is incorrect. It is not: Group 18 is the current IUPAC numbering, while Group 0 is the historical designation used in Mendeleev’s arrangement. Mendeleev also speculated about hypothetical gases lighter than hydrogen, ideas later associated with his unaccepted ether theories.