Valency is defined as" The number of electrons that an atom wants to lose ,gain or share is called its valency". Example: Sodium(Na) atom loses one electron .Its's valency is '1
Valence is typically the number of electrons needed to fill the outermost shell of an atom. Because exceptions exist, the more general definition of valence is the number of electrons with which a given atom generally bonds or number of bonds an atom forms. (Think iron, which may have a valence of 2 or a valence of 3.)
The IUPAC formal definition of valence is the maximum number of univalent atoms that may combine with an atom. Usually, the definition is based on the maximum number of either hydrogen atom or chlorine atoms. Note the IUPAC only defines a single valence value (the maximum), while atoms are known to be capable of displaying more than one valence. For example, copper commonly carries a valence of 1 or 2.
Examples: A neutral carbon atom has 6 electrons, with an electron shell configuration of 1s22s22p2. Carbon has a valence of 4 since 4 electrons can be accepted to fill the 2p orbital.
Common Valences
Atoms of elements in the main group of the periodic table may display a valence between 1 and 7 (since 8 is a complete octet).
Group 1 (I) - Usually displays a valence of 1. Example: Na in NaCl
Group 2 (II) - Typical valence is 2. Example: Mg in MgCl2
Group 13 (III) - Usual valence is 3. Example: Al in AlCl3
Group 14 (IV) - Usual valence is 4. Example: C in CO (double bond) or CH4 (single bonds)
Group 15 (V) - Usual valences are 3 and 5. Examples are N in NH3 and P in PCl5
Group 16 (VI) - Typical valences are 2 and 6. Example: O in H2O
Group 17 (VII) - Usual valences are 1 and 7. Examples: Cl in HCl