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

Hint:To answer this question, you must recall the trends properties in the modern periodic table. The periodic table is arranged in such a way so as to show various properties of atoms in a set trend. It is possible since the atoms are arranged on the basis of the atomic numbers of atoms and valency depends on atomic number and determines many properties. The atomic number increases along the period.

Complete answer:
To study the trend of valency, we must be familiar with the concept of valency. The valency of an atom is the number of electrons present in the valence shell of the atom. These electrons are the most loosely held electrons in the atom and thus determine the properties of the element. Each atom aims to attain a stable electronic configuration which is similar to that of a noble gas. This electronic configuration is attained by losing, gaining or sharing electrons.
Valency of an element gives the combining capacity of the element. Valency does not always necessarily represent the number of valence electrons. For valence electrons less than 4, the valency is equal to the number of valence electrons and for valence electrons more than 4, the valency is given by subtracting the number of electrons from 8.
Hence, the valency first increases and then decreases as we move from left to right in a period.

Note:
The modern periodic table has 18 groups and 7 periods in which the atoms are arranged in the increasing order of their atomic numbers which denote the number of electrons in the element. The number of electrons, specially the valence electrons, decides all the chemical properties of a certain atom.