Question

why hydrogen can be placed in 1st group and 17th group both?​

Answer 1

Hydrogen is placed above group in the periodic table because it has ns1 electron configuration like the alkali metals. However, it varies greatly from the alkali metals as it forms cations (H+) more reluctantly than the other alkali metals.

Answer 2

due to dual nature of H. in H atom 2s orbital contains 1 electron ....thus it can lose the electron to behave as alkali metals(Grp-1)or gain an electron to attain a negative charge like halogens(Grp-17) .Hence hydrogen can be placed in 1st group as well as 17 th group.

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