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

As we move from left to right across a period in the periodic table, the valency of elements tends to change in a predictable pattern. Valency is a measure of an element's combining capacity or the number of electrons an atom can gain, lose, or share to achieve a stable electron configuration.

Here are the general trends in valency as we move from left to right through a period:

Group 1 (Alkali metals): Elements in this group have one valence electron, and they tend to lose that electron to achieve a stable electron configuration like the noble gas in the previous period. So, their valency is usually +1.

Group 2 (Alkaline earth metals): Elements in this group have two valence electrons and typically lose both electrons to attain a stable configuration like the nearest noble gas. Therefore, their valency is usually +2.

Transition metals: Elements in the middle of the periodic table (Groups 3 to 12) are transition metals. They have variable valency since they can lose different numbers of electrons from their outermost shell to form various positive ions.

Group 13 to Group 17: The valency of elements in these groups varies as they move from left to right. Group 13 elements (Boron group) usually have a valency of +3; Group 14 elements (Carbon group) can have valencies of +4 or -4; Group 15 elements (Nitrogen group) typically have a valency of -3; Group 16 elements (Oxygen group) have a valency of -2, and Group 17 elements (Halogens) have a valency of -1.

Group 18 (Noble gases): Noble gases have full valence shells, and their electron configuration is already stable. As a result, they have a valency of 0, meaning they don't readily gain, lose, or share electrons with other elements.

In summary, as you move from left to right across a period, the valency generally increases for the first few groups (due to the increasing number of valence electrons) and then becomes variable for the transition metals. Later in the period, the valency tends to decrease and stabilize as the elements approach the noble gases with full outer electron shells.