The number of electrons gained, lost, or shared by the atom of an element, so as to complete its octet or duplet is called the valency of the element. Valency is also known as the combining capacity of the atoms of the element i.e. their tendency to react and form molecules with atoms of the same or different elements.

How are electrons distributed in different orbits (shells)? The distribution of electrons in different orbits of an atom was suggested by Bohr and Burry. The distribution or arrangement of electrons in different shells of an atom is called the electronic configuration of the element. They suggested:-

�    The maximum number of electrons present in a shell is given by the formula �2n2� where �n� is the orbit number i.e. 1, 2, 3�. and so on. So in the K Shell, i.e. the 1st orbit, the number of electrons comes out to be = 2*12 which is equal to 2.

In the L Shell, i.e. the 2nd orbit, the number of electrons will be =2*22=8. In the M Shell, i.e. 3rd orbit, the number of electrons will be =2*32=18. In the N Shell, i.e. 4th orbit, the number of electrons will be =2*42=32

�    The maximum number of electrons that can be accommodated in the outermost orbit is 8.
�    Electrons do not enter in a given shell, until and unless the inner shells are filled i.e. the shells are filled in a stepwise manner.

Valency 

We know that in an atom, the electrons revolve around the nucleus in the fixed orbits. The electrons present in the outermost shell of an atom are known as the valence electrons and the outermost shell is known as the valence shell. The outermost shell of an atom can accommodate a maximum of 8 electrons and the atom tends to react in order to complete its outermost shell i.e. it tries to complete 8 electrons in its outermost shell or attain the structure of the nearest noble gas.

Noble gases are stable gases i.e. they are chemically inert and have 8 electrons in their valence shell, except for the atom which has only K shell which can accommodate a maximum of 2 electrons. All other atoms have less than 8 electrons in their valence shell and hence to attain stability, atoms of these elements lose, gain or share electrons with other atoms.

For elements having valence electrons equal to 1, 2, 3, and 4 valency is equal to the number of valence electrons. Hydrogen has an atomic number of 1, the number of electrons is 1, and so the number of electrons in its outermost shell is 1 so its valency is also 1.

Atomic No of Beryllium is 4; the number of valence electrons is 2 so its valency is also 2. The number of electrons in Boron is 5; the number of valence electrons is 3, so its valency is also 3. Similarly, in the case of Carbon, the number of outermost electrons are 4, so its valency is also 4. But for elements having valence electrons equal to 5,6, or 7, their valency is equal to the number of electrons to be added so that the valence shell has 8 electrons i.e. Valency is equal to 8 minus the number of valence shell electrons.

In the case of Nitrogen, the number of valence electrons is equal to 5 so its valency is equal to 3.
While in case of oxygen, no of valence electrons is equal to 6 and its valency is equal to 8 minus 6 which is equal to 2.

Similarly, fluorine has 7 valence electrons, so its valency is 1. In the case of noble gases, where the outermost shell already has 8 electrons i.e. their octet is already complete, have their valency equal to zero. So if we know the atomic number of an element, we can easily find out its valency. The steps will be:-

1. If atomic number of the atom is known.
2. Write the number of electrons present in the atom.
3. Then write down its electron configuration.
4. Now find out the number of valence electrons.
5. Then write down the valency equal to number of valence electrons where valence electrons are equal to 1, 2, 3 or 4. Or it is equal to 8 minus number of valence electrons where number of valence electrons is equal to 5, 6 or 7.

Read More: Structure of an Atom: Electrons, Protons, Neutrons | Sub Atomic Particles