Engineering Chemistry S Datta

VALENCY AND CHEMICAL BONDING 15
This sort of attractions give the covalent bond between two atoms a partial ionic character.
Partial ionic character of a covalent bond is calculated as follows:
When electronegativity values (x) of the two atoms A and B are same, i.e., x A
= x B
, the
bond A – B is 100% covalent. When x A
≠ x B
, the bond gains partial ionic character and the
molecule AB becomes polar, viz. HCl. Greater the difference (x A
– x B
), greater is the ionic
character of the bond. The relation is represented by the equation —
% ionic character = 100 1−exp 1− 1 ( xA
− xB)
4
= 16(x A
– x B
) + 3.5 (x A
– x B
) 2
Calculation of partial ionic character of HF molecule:
% ionic character in HF molecule = 16(4.0 – 2.1) + 3.5(4.0 – 2.1) 2
= 43%.
Hydrogen Bond
Hydrogen atom has got only one electron. So, it can combine with one atom of a
monovalent atom, viz F. If the second atom is electronegative, then electron pair of the bond is
shifted towards the other atom, i.e., F, due to its greater electron attraction power. As a result,
the hydrogen will gain the positive charge partially but the electron will not be detached totally
from the hydrogen atom. In this condition, if the charged hydrogen atom comes in contact with
strongly electronegative element of the other molecules (hydrogen of HF), they are attracted
by the hydrogen (H +δ ) remaining in the previous molecule and a ‘so-called’ valency is formed as
if hydrogen acts a bridge between two electronegative elements. This sort of bond is known as
‘hydrogen bond’. This type of bond is not very strong. The bond energy is less than that of
covalent bonds.
Example of hydrogen bonding in HF molecules:
L
NM
+ – + – + – + –
H – F ... H – F ... H – F ... H – F
hydrogen bond
Highlights:
• Conditions of hydrogen bond —
(i) Hydrogen should be attached to an electronegative element of a covalent bond.
(ii) The atomic radius of the electronegative elements should be small.
• Hydrogen bond is seen when hydrogen is attached to electronegative elements, like
O, N and F (i.e., when atomic radii are small).
Hydrogen Bondings
There are two types of hydrogen bonding: (a) Intermolecular (b) Intramolecular.
(a) Examples of intermolecular hydrogen bonding :
(i) ...... H–F ...... H–F ...... H–F ...... H–F......
(ii) HO
Covalent
bond
H
HO
H
HO
H
Hydrogen
bond
HO
H
RST
2
O UVW QP