Final answer:
The discovery of noble gases led to the addition of a new group (Group 18) to Mendeleev's periodic table, establishing them as unreactive elements with filled valence shells, but later research showed they could form compounds under certain conditions.
Step-by-step explanation:
The discovery of the noble gases changed Mendeleev's periodic table by adding a new group, specifically Group 18. When Sir William Ramsay discovered the noble gases between 1894 and 1898, he initially placed them between the halogens (Group 17) and the alkali metals (Group 1). It wasn’t until the work of H. G. J. Moseley in 1913, who proposed the arrangement of elements based on atomic number rather than atomic mass, that the noble gases found their rightful place on the periodic table. The confirmation of noble gases as a distinct group further evolved with the realization that they were not completely unreactive, as originally thought, a fact demonstrated by Dr. Neil Bartlett's research in 1962.