Question

in the first period hydrogen and helium are not placed adjacent to each other why?

Answer 1

it is coz hydrogen has dual behaviour of both first group of alkali metal and of fluorine family ... as it is so hydrogen is placed in first group or 17 group .. but ut was placed in first group and far from helium belonging to 0 group elements coz it resembles more of properties of first group..
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it was the original reason why they are not side by side .and there are many other reasons as you know like electron affinity and ionization energy also satisfy the answer

. but as per modern periodic table of arihant and cengage series of chemistry books both hydrogen and helium are placed side by side as given in pic

Answer 2

Hey dear,

● Explaination -
- Though elements are arranged in increasing order of atomic masses, selection of appropriate group is determined by their chemical properties.
- Hydrogen has one valence electron in outermost shell & it's electropositive so its added to group of alkali metals.
- Due to completely filled valence shells & its inert activity Helium is put with inert gases.
- Therefore, H & He are far away from each other in modern periodic table.

Thanks...