Valency (oxidation number) is characterizing the ability of an atom to combine with another atom.
It is based on the number of electrons that atom participates in chemical bond formation and varies depending on the atom and chemical group.
Valency is determined by the number of electrons involved in chemical bonds.
The number of main group indicates the maximum valency of elements. Valency may relate to hydrogen and oxygen. Valency of elements relative to hydrogen increases in period from group IA to group IVA, and then decreases. For elements of groups V-VII ,valency to hydrogen is determined by using the relationship: valency = 8 - no. group.
Electro valency represents valency of elements which are converted to ions and is equal to the number of assigned or accepted electrons.
Covalency represents the valency expressed by the number of electrons which an atom puts together with electrons of another atom. Covalency is denoted by roman numerals, written in parentheses, on the right side of the symbol. For example: H (I), A (II).
Valency of elements is considered between two elements.
Valency, which is also called valence, indicates the the number of atoms of element that combine with number of atoms of other elements to form molecules of chemical compounds. The exact definition of valence has been modified by scientists over a period. Initially it was defined as number of hydrogen atoms that combine with atoms of different elements. As per this definition, the valence of hydrogen itself is taken as 1. For example, as one atom of oxygen combines with two atoms of hydrogen to produce H2O (water), the valence of oxygen is 2.
Subsequently valence was defined based on charge of ionized atom. Finally, the valence is now defined based on chemical links or bonds that an atom of an element forms with other elements. As atoms of many elements can combine in several different ways resulting in more than one valence for them.
From this above definition of valence it is clear that valence of an element can be best determined by understanding the structure of molecules of their compounds.
The combining ability of an element to combine with other element.it is determine by the number of electron in valence shell of each atom of an element.