CHEMICAL BONDING

I NT RO DU CT IO N

(a) It is well known fact that except for inert gases, no other element exists as independent atoms under
ordinary condition.

(b) Most of the elements exist as molecules which are cluster of atoms. How do atoms combine to form
molecules and why do atoms form bonds? Such doubts will be discussed in this chapter.

(c) A molecule will only be formed if it is more stable and has a lower energy, than the individual atoms.

Chemical Bond :

(a) A force that acts between two or more atoms to hold them together as a stable molecule.

(b) It is union of two or more atoms involving redistribution of e– among them.

(c) This process accompanied by decrease in energy.

(d) Decrease in energy  Strength of the bond.

(e) Therefore molecules are more stable than atoms.

CAUSE OF CHEMICAL COMBINATION

1. Tendenc y to acquire mi nimum energy :

(a) When two atoms approaches to each other-

Nucleus of one atom attracts the electron of another atom.

(b) Two nuclei and electron of both the atoms repells each other.
Potential Energy diagram
(c) If net result is attraction, the total energy of the system (molecule) decreases and a chemical bond forms.

(d) So Attraction  1/energy  Stability.

(e) Bond formation is an exothermic process

2. Octet rule

Octet rule was given by Lewis & Kossel

Atoms Combines to complete an octet of electrons in their outer most orbit. Complete orbital represents to get most
stable state. Hence all atoms have a tendency to acquire octet (s2p6) configuration in their outermost orbit.

The octet may be complete in following manner:

Complete transfer of electrons from one atom to another.

Ex. NaCl, CaCl2 & MgO etc. (Ionic Bond)

Sharing of electrons between atoms.

(a) Sharing of equal number of electron between two atoms.

Ex. Cl2, N2, O2 etc., (Covalent bond)

(b) Sharing of electron pair given by only one atom

Ex. [NH3  H+] & NH3  BF3 (Co-ordinate Bond)

 EXCEPTIONS OF OCTET RULE
1. Tra nsit ion metal ions
Cr 3+ Mn 2+ Fe 2+
[Ar]3d 3 [Ar]3d 5 [Ar]3d 6
[2, 8, 11] [2, 8, 13] [2, 8, 14]
2. Pseudo inert gas configuration [s 2 p 6 d 10 ]
Zn 2+ Cd 2+
[Ar]3d 10 [Kr]4d 10
3. Contraction of octet (incomplete octet)
Be F2 B
——
F
— 3
Al Cl3 B
—— —
Cl3 F
×
|
..
(4e) (6e) (6e) (6e)
B
. ×F (6 Electrons)

|
|
×
4. Expansion of Octet (due to empty d-orbitals) F

PCl 5 SF 6 ClF3 ICI5 IF7 F F

    
F S F
(10e) (12e) (10e) (12e) (14e)
5. Odd electron species F F
(12e)
Ex. NO, NO2, ClO2 etc.

O
N , N O
O
6. Compounds of Noble gases
Noble gases which have already completed their octets (or douplet in case of He.) should not form compounds.
However, their compouinds like XeF2, XeF6 & KrF2 etc., have been actually prepared.
CL ASSIFICATION OF BONDS

A T T R A C T IV E F O R C E S

STRONG BOND WEAK INTERACTION

Ionic Covalent Co-ordinate Metallic Hydrogen Vander waal's

bond bond bond bond bond Interaction
COVALENT BOND
(a) A covalent bond is formed by the mutual sharing of electrons between two atoms of electrolnegativity
elements to complete their octet.(Except H which completes its duplet)
× ×
H  H H – H









×






×
  ×  O= O

  ××  N N
× ×








× ×



H2 molecule O O N N
(b) The shared pair of electrons should have opposite spins, and are localised between two atoms
concerned.
 (c) Shairing of electrons may occurs in three ways –
No. of electrons shared Bonded Electron pair Bond.
between two atoms
2 1 Single bond (–)
4 2 Double bond (=)
6 3 Triple bond ()
Ex. 
H –– N –– H {Three single bonds (not triple bond}

H
N  N Triple bond. (not three single bond) O = O (Double bond) H – O – H (Two single bonds.)

CO-ORDINATE BOND

(a) It is a covalent bond in which the shared electron pair come from one atom is called coordinate bond.
(b) Necessary conditions for the formation of co-ordinate bond are -
(i) Octet of donor atom should be complete and should have atleast one lone pair of electron.
(ii) Acceptor atom should have a defficiency of at least one pair of electron.
(c) Atom which provide electron pair for shairing is called donor.
(d) Other atom which accepts electron pair is called acceptor. That is why it is called donor-acceptor or
dative bond.
H F
 
H–N: + B–F  NH3 BF3
 
H F
BF3 is electron defficient compound.
H3N  NH
 3

Metal co-ordinate compounds - [Cu(NH 3) 4] +2  Cu+2

H3N  NH
 3
+
H

Ex. NH4+ ; H3N + H+ H – N –H

(Lowry-Bronsted acid) H
(e– acceptor)

H H






+
H3O ; O + H +
 O  H+
H H

N2O ; N  N O

 Note : Compounds in which Ionic, covalent and co-ordinate bonds are present, are as follows -
NH4Cl, CuSO4.5H2O, K4[Fe(CN)6], KNC, KNO3, etc.