Why is the valency of group IA as well as VIIA with respect to hydrogen one?
The valency of each is one because each can bond with one hydrogen atom.
Valence electrons are the outermost electrons of an atom.
The valence or valency refers to the ability of an element to combine with other atoms. One definition is:
Valence is the number of hydrogen or chlorine atoms that can combine with an atom of the element.
The elements in Group 1 are H, Li, Na, K, etc.
They all have one valence electron that they can share with or donate to H atoms.
They form compounds like H₂, LiH, NaH, KH, etc. In this way, they complete their valence shells.
So the valency of Group 1 elements is 1.
The important elements in Group 17 are F, Cl, Br, and I.
They all have one valence electron that they can use to share with H atoms or Cl atoms.
They form compounds like HF, HCl, HBr, and HI. In this way they complete their valence shell octets.
So the valency of the Group 17 elements is 1.