Which elements are exception in classification of periodic table.

1.	Which elements are exception in classification of periodic table.
Answer» Solution :Two elements like hydrogen and helium are observed as an exception . Place of hydrogen in periodic table : Hydrogen contain le in ls orbital so it must be situated in group-1. Hydrogen ACCEPT `1 e^(-)`and from stable structure like helium. So, it must be situated in group-17 But there is same DIFFERENCE observed between hydrogen and ist group and hydrogen and `17^(th)`group so hydrogen atom is situated in separately in periodic table. Place of helium in periodic table . Helium element having `1 ^(ST)`electron configuration so it must be present in s-block. But its positioning in the p-block along with other group 18 elements is Justified. Because it has a completely filled valence shell `(1 s^(2))` and as a RESULT, exhibits properties CHARACTERISTIC of other noble gases.

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