The variation in valency along a period (horizontal row) in the periodic table can be explained by the electronic configuration of the elements. Valency refers to the combining capacity or the number of electrons that an atom of an element can gain, lose, or share to form chemical bonds with other atoms.
As you move from left to right along a period, the number of protons and electrons in the atom increases, filling up the same principal energy level. However, the increase in effective nuclear charge (the attractive force experienced by the outermost electrons from the nucleus) outweighs the shielding effect of inner electron shells, resulting in a stronger pull on the outermost electrons.
The increase and decrease in valency along a period can be understood based on the following general trends:
Valency increases from left to right: In the early part of a period, elements have fewer electrons in their outermost energy level, making it easier to lose or share those electrons to achieve a stable electron configuration. These elements tend to have low ionization energies and readily lose electrons to form positive ions. For example, the alkali metals (Group 1) have one valence electron and tend to lose it, forming monovalent cations.
Valency decreases after reaching a maximum: As you move towards the right side of a period, the elements start approaching the noble gases (Group 18) with stable, completely filled electron configurations. These elements have little tendency to either lose or gain electrons, resulting in a valency of zero. They are chemically inert. For example, the noble gases have completely filled valence shells and rarely form chemical bonds.
The decrease in valency occurs because the increasing number of electrons and the approaching full electron shell make it energetically unfavorable for the atom to lose or gain additional electrons. Instead, elements in this region tend to share electrons with other atoms to achieve a stable octet (or duet, in the case of hydrogen and helium) in their valence shells.
It's important to note that while these trends generally hold true for the elements across a period, there can be exceptions and variations based on specific elements and their electronic configurations.