Question

give reason why noble gases are placed in separate group in Modern Periodic Table

Answer 1

The groups are elements with the same number of electrons in the outer shell at least for the more common elements. The noble gases like helium,neon. argon, krypton and xenon are placed in a separate group because these areinert ( un-reactive ) and present in very low concentration in the atmosphere.
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Answer 2

it is because the shells of the noble gases are fully filled and they are not reactive towards any other elements. they do not lose or gain or share their electrons with other elements