Question

what is the hydrogen placed in 1st group ​

Answer 1

it is placed so.. because it contains properties of halogens and also (group one) alkali metal

but it has only one electron that why it is place in 1st group

Answer 2

Here is your answer

In Mendeleev's periodic table Hydrogen is placed in 1st group has no proper explanation as the elements were arranged according to hydrides and oxides and increasing atomic mass.

But in Modern Periodic table Hydrogen in placed in 1st group because it is arranged according to electropositivity and H2 is an electropositive element.

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