Question

Why is the valency of group IA as well as VIIA with respect to hydrogen one?

Answer 1

To answer the question we need to understand the concept of valency. Valency is the measure of the combining capacity of an element when it forms compounds or molecules. Every element wants to attain a stable valence shell electronic configuration. Valency is the measure of electrons lost or gained by an element to attain the stable configuration.

Complete answer:
Let us discuss some characteristics of the valency of elements-
(A)All the elements of a group have the same valency.
(B)The valency of the s-block elements (Alkali metals and alkaline earth metals) is calculated using the valence electrons of the atom or eight minus the number of valence electrons.
(C)The valency of the d-block elements or the transition elements is not fixed. They show variable valencies because not only the s electrons but also the penultimate d electrons participate in bonding.
(D)The f-block elements or the inner transition elements also show variable valencies because of the participation of both s as well as f electrons in bonding.
(E)The noble gases have a valency of 0 since they are already stable and need no electrons.
There is another definition of valency that is used lately-
Valency is defined as the number of univalent atoms like Hydrogen and Chlorine that combine with the element under consideration.
In the question, we are asked why the valency of both groups IA and VIIA is one with respect to hydrogen. The reason is as follows-
Group IA contains elements - $Lithium(Li),Sodium(Na),Potassium(K),Rubidium(Rb),Caesium(Cs)\& Francium(Fr)$
Let us look at the electron count in the Alkali metals-

Element	Electronic configuration
Lithium(Li)	2,1
Sodium(Na)	2,8,1
Potassium(K)	2,8,8,1
Rubidium(Rb)	2,8,8,18,1
Caesium(Cs)	2,8,8,18,18,1
Francium(Fr)	2,8,8,18,18,32,1

We see that Alkali metals (group IA) can easily lose an electron from the valence shell resulting in a stable electronic configuration. These elements combine with Hydrogen to form $LiH, NaH, KH$ etc. Hence it has a valency of 1 with respect to hydrogen.
Group VIIA includes the following elements-
$Fluorine(F),Chlorine(Cl),Bro\min e(Br),Iodine(I)\& Astatine(At)$
Let us look at the electron count in the group VIIA (Halogen family)-

Element	Electronic configuration
Fluorine(F)	2,7
Chlorine(Cl)	2,8,7
Bromine(Br)	2,8,18,7
Iodine(I)	2,8,18,18,7
Astatine(At)	2,8,18,18,32,7

We see that these elements require one electron to attain a stable valence shell configuration. These elements combine with hydrogen to form $HF, HCl, HBr$ etc. Hence it has a valency of 1 with respect to hydrogen.

Note:
Hydrogen also has a valency of 1 since it has an atomic number 1 and hence the presence of 1 electron in its atomic shell. However, the position of hydrogen is quite uncertain in the periodic table. It can either lose the single electron to form a cation showing similarity to alkali metal or gain an electron just like the halogen family. Hence to maintain the symmetry of the periodic table it is kept above the alkali metals.