Question

How does valency vary from when we move from left to right through a period?

Answer 1

As we know in the periodic table total 118 elements are present now. These elements are arranged in rows and columns. These rows and columns are called periods and groups. This arrangement of elements depends on the atomic number of the atoms. Valency is the important concept in the periodic table. The valency of the element is the main factor for formation of molecules. The valency can be positive, negative, and zero also.

Complete step by step answer:
We have studied that valency is nothing but the extra electron or deficient electron of the atom that attains stability. In every period from left to right, the valency first increases and then decreases. In every period, valency starts from $1$ on the left side and moving to the right side, it increases up to $4$ and then decreases. In the periodic table, the number of electrons in elements from left to right increases from $1$ to $8$. In inert gases, the last right group valency of these groups is zero, because elements in this group are already stable and don't need to donate or accept electrons to attain the stability.

Note: We must remember, if the valency of an electron is positive, it implies that the element has extra electrons after attaining stability. For example, in the first group all the elements have a $+ 1$ valency state. All the elements in this group have one extra electron after attaining stability. Same is with second and third group as they have valency numbers of $+ 2$ and $+ 3$. In group 16 and 17, the valency of the element becomes negative. Because elements in these groups are deficient of one or two electrons to attain stability. In group 17, the valency of the element is $- 1$ because here the element requires one electron to attain the stability.