Which group of elements could be placed in mendeleev's periodic table without disturbing the original order ? Give reason.

Explanation

The node gases could be placed in a new group without disturbing the existing order because they were very inert and present in extremely low concentration in out atomosphere.
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 (Rn) were not known. When these gases were discovered much later, these were placed in new group called the zero group.