Why for the formation of a covalent bond both the atoms should have high electronegativity? Why should the electronegativity difference between b/w the combining atoms involved in covalent bond should be little or negligible because if the electronegativity difference between the combining atoms is high then too polar covalent compounds are formed?please give appropriate reason and explain in an easy language.

Asked by 21janhvi.verma | 6th Jun, 2018, 10:34: AM

Expert Answer:

After observing your questions it is clear that your idea about electronegativity and chemical bonding is not cleared.
Lets first understand it.
Electronegativity: It is the property of an atom of an element to attract or pull electrons towards itself.
Now, let's apply this concept to chemical bonding. 
Always remember two points:
1) If the electronegativity difference between combining atoms is zero or negligible then it will form a covalent compound
2) If the electronegativity difference between combining atoms is high then it will form an ionic bond.
There is a case in covalent compounds,i.e., Polar covalent compounds
let's understand when a covalent compound is polar,
This happens when bonded atoms are dissimilar atoms with little difference in their electronegativity.
For example HCl ( electronegativity of H=2.2 and Cl = 3.16) 
The difference in their electronegativity is 3.1-2.2 = 0.9 which is more than 0, hence the nature of bond will be polar
Since Cl is more electronegative than hydrogen it will attract the shared pair of electrons towards itself, thereby developing a slight negative charge (δ−) on Cl and (δ+) charge
But when HCl is in its gaseous form it will never dissociate, but when we dissolved it into a polar solvent it dissociates to form ions.
H C l space rightwards arrow over leftwards arrow with H subscript 2 O on top space H to the power of plus space plus space C l to the power of minus
This happens because the OHion from water molecule will attract the H+ ion of HCl hence as both are oppositely charged they attract towards each other and bond between H-Cl is broken and dissociates to form ions.
Points to remember: 
* Covalent compounds do not form ions 
* Polar covalent compounds do not form ions unless they come in contact with or dissolved in the polar solvent.
* Ionic compounds are more stable than covalent compounds.
* Ionic compounds are in ionic form in their crystals but when they are in contact with or dissolved in the polar solvent they dissociate into ions. 

Answered by Ramandeep | 6th Jun, 2018, 02:13: PM