Concept of valency and valence electrons

Atoms of elements have natural tendency to combine with atoms of other elements. When two or more atoms of same or different elements combine together and form a stable system, it is called a ’molecule.’ For example, an atom of sodium and chlorine combine to form a molecule of sodium chloride, an atom of Hydrogen and Fuorine combine to form a molecule of hydrogen fluoride. Each atom has some combining power or capacity with other atoms. This combining capacity of the atom of an element is called valency. The valency of hydrogen and chlorine is assumed to be one. The valency of other elements is found out with respect to the valency of hydrogen or chlorine.

The number of hydrogen atoms or chlorine atoms which combine with one atom of the element is called the valency of the element.

In modern language, valency of an element is also defined as follows :

The number of electrons that are shared, lost or gained by an atom during a chemical reaction is called as the valency of the element.

The outermost shell of the atom is called the ’valence shell’ and the electrons present in that shell are called the ’valence electrons’. They determine the valency of the element.

 

Types of valency

There are two types of valency –electrovalency and covalency

Electrovalency –The number of electrons lost or gained by an element to acquire the nearest inert gas configuration i.e. stable configuration is called electrovalency of that atom. For example, atoms of Li, Na, K lose one electron so their electrovalency is 1, atoms of Mg, Ca, Ba lose two electrons , so their electrovalency is 2. The atoms of O, S gain two electrons so their electrovalency is 2 and the atoms of Cl, Br, I gain one electron so their electrovalency is 1.

The atoms of inert gases have completely filled outermost orbit. So they do not lose, gain or share electrons. So their valency is zero.

Covalency –The number of electrons shared by an atom of one element with the atom of same or different element is called the covalency of the atom.

For example, oxygen atom shares two electrons with two H atoms to form H2O. So the covalency of oxygen is 2.

A silicon atom shares four electrons with four Cl atoms to form SiCl4 . So the covalency of silicon is 4.

While forming a stable molecule or during a chemical reaction, an atom tries to complete its octet or duplet. So the number of electrons lost, gained or shared by an atom to complete its valence shell or octet is also taken to be the valency of the atom.