Answer:
The noble gases could
be placed in a new group without disturbing the existing order because they
were very inert and present in extremely low concentrations in our atmosphere.
When Mendeleev gave
his periodic table, at that time noble gases such as helium (He),neon (Ne),
argon (Ar), krypton (Kr), xenon (Xe) and radon (Fin) were not known. When these
gases were discovered much later, these were placed in new group called the
zero group.
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