Question

explain with reason the variation of valency in a period from left to right in the modern periodic table

Answer 1

Valency of an element is defined by the number of valence electrons or outermost electrons present in the atom of the element.
On moving from left to right in each period, the valency of element increases from 1 to 4 and then decreases to zero.
Reason:
In each period, only one electron will be added in same outer most shell as we move from left to right.
So Atomic number changes.
Electronic configuration changes hence the reactivity of the elements changes hence valency also changes.