Question

What is difference between valency and co valency

Answer 1

What is Valency ??

In order to write chemical formulae we need to understand how elements combine with each other to form molecules or ionic compounds. Ionic compounds are substances made from the combination of charged ( positive and negative) particles.

Valency refers to the ability of an atom or a group of chemically bonded atoms to form chemical bonds with other atoms or groups of atoms. 
The valency of an element is determined by thenumber of outer shell (valence) electrons.

For Example,
1 . SODIUM
Atomic Number - 11
Electronic Configuration - K-2 , L-8 , M-1

Atom of every element wants to have eight electrons in it's valence shell. As you can see above sodium has one electron in it's valence shell. In order to get a completely filled outermost shell it has to lose this one electron. So, it will have a valency of 1.

2 . SULPHUR
Atomic Number - 16
Electronic Configuration - K-2, L-8, M-6
 
As, sulphur has six electrons in it's valence shell it has to gain 2 more electrons to have a full valence shell and it will have a valency of 2.

Covalency ??

The covalency of an atom is the number of electrons it shares with other atoms when forming chemical bonds. Usually, but not always, it is the number of covalent bonds it forms with other atoms.

Simply it can be considered as number of electrons pairs an atom can share to other atoms to form bonds . consider oxygen it contain 2 valence electrons so it can form 2 bonds and its Covalency is 2 . Similarly nitrogen is having 3 valence electrons so it can form 3 bonds and itsCovalency is 3

To determine the covalency, you need to draw the Lewis structure of the molecule and count the number of shared electron pairs.