A — H - - - B
A: nitrogen/oxygen/fluorine
B:fluorine/oxygen/nitrogen/ chlorine
N/O/F are highly electro-negative elements. When they are bonded to a hydrogen atom to form a covalent bond then the electron of the bond are attracted towards N/O/F due to which partial positive charge develops on hydrogen atom and this partial positive charge on hydrogen forms a bond with another electro-negative atom having lone pair. This particular bond is known as hydrogen bond.
Hydrogen bond or H-bond is weaker than covalent bond and acts as a bridge between two atom which holds one by covalent bond and the other by H-bond.
H-bond is represented by dotted line (- - -) while covalent bond is represented by solid line (— ).
Attractive force which binds H atom of one molecule with electro-negative atom of another molecule is defined as H-bond.
Magnitude of H-bond depends upon physical state of the compounds. It is maximum in solids and minimum in gases. So it influences structure and property of compounds to a large extent.
Bond energy of H-bond is usually low but there are examples in which H-bond energy is nearly 220 kJ/mole (equivalent to covalent bond) hence we classify H-bond into three categories:
(i) Weaker H-bond: bond energy < 25 kJ/mole
(ii) Moderate H-bond: 25 < bond energy < 40 kJ/mole
(iii) Stronger H-bond: bond energy ~ 220 kJ/mole (in HF2-) in this case covalent bond and H-bond are nearly same in strength.
Condition for formation of covalent bond:
A — H - - - B
A: →it should be highly electro-negative
B: →it should be electro-negative →have lone pair →small in size
Strength of H-bond is directly proportional to the electro-negative of A and inversely proportional to the electro-negative of B.
Types of H-bond:
(i) Inter-molecular H-bond
(ii) Intra-molecular H-bond
