According to the modern periodic table, across the period the valency of elements-

In the modern periodic table, across the period the elements have the same outermost shell which means the electrons are added to the same outermost shell. The valency of an element is the electrons needed for that element to complete its outermost shell octet.

Moving across a period, when electrons are added the valency also starts increasing from 1 to 4 and then again starts decreasing from 4 to 1.

Hence, across a period, the valency trend first increases and then decreases.