CHEMICAL BONDING | Lucent for GK - UPSC PDF Download

 Page 1


CHEMICAL BONDING
INTRODUCTION
(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 . Tendency to acquire minimum energy :
(a) When two atoms approaches to each other-
Nucleus of one atom attracts the electron of another atom. 
Potential Energy diagram
(b) Two nuclei and electron of both the atoms repells each other.
(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 (s
2
p
6
) configuration in their outermost orbit.
The octet may be complete in following manner:
Complete transfer of electrons from one atom to another.
Ex. NaCl, CaCl
2
 & MgO etc. (Ionic Bond)
Sharing of electrons between atoms.
(a) Sharing of equal number of electron between two atoms.
Ex. Cl
2
, N
2
, O
2
 etc., (Covalent bond)
(b) Sharing of electron pair given by only one atom
Ex. [NH
3
 ?? H
+
] & NH
3
 ?? BF
3
 (Co-ordinate Bond)
Page 2


CHEMICAL BONDING
INTRODUCTION
(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 . Tendency to acquire minimum energy :
(a) When two atoms approaches to each other-
Nucleus of one atom attracts the electron of another atom. 
Potential Energy diagram
(b) Two nuclei and electron of both the atoms repells each other.
(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 (s
2
p
6
) configuration in their outermost orbit.
The octet may be complete in following manner:
Complete transfer of electrons from one atom to another.
Ex. NaCl, CaCl
2
 & MgO etc. (Ionic Bond)
Sharing of electrons between atoms.
(a) Sharing of equal number of electron between two atoms.
Ex. Cl
2
, N
2
, O
2
 etc., (Covalent bond)
(b) Sharing of electron pair given by only one atom
Ex. [NH
3
 ?? H
+
] & NH
3
 ?? BF
3
 (Co-ordinate Bond)
EXCEPTIONS OF OCTET RULE
1 . Transition 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)
— —
BeF
2 —
BF
3 — —
AlCl
3 —
B Cl
3
(4e) (6e) (6e) (6e)
4 . Expansion of Octet (due to empty d-orbitals)
PCl
5
SF
6
ClF
3
ICI
5
IF
7
? ? ? ? ?
(10e) (12e) (10e) (12e) (14e)
5 . Odd electron species
Ex. NO, NO
2
, ClO
2
 etc.
N
O
O
    ,      
N 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 XeF
2
, XeF
6
 & KrF
2
 etc., have been actually prepared.
CLASSIFICATION 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
H m olecule
2
          
?
?
?
?
? ? ? ?
?
?
?
?
?
? ? ? ?
?
?
? O = O
O O
×
×
×
×
×
×
×
×
       
?
?
?
?
?
? ?
?
?
?
? ?
? N  N ?
N N
×
×
×
×
× ×
(b) The shared pair of electrons should have opposite spins, and are localised between two atoms
concerned.
F
|
|
|
.
.
.
B F
F
(6 Electrons)
×
×
×
F
F
F F
F
F S
(12e)
Page 3


CHEMICAL BONDING
INTRODUCTION
(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 . Tendency to acquire minimum energy :
(a) When two atoms approaches to each other-
Nucleus of one atom attracts the electron of another atom. 
Potential Energy diagram
(b) Two nuclei and electron of both the atoms repells each other.
(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 (s
2
p
6
) configuration in their outermost orbit.
The octet may be complete in following manner:
Complete transfer of electrons from one atom to another.
Ex. NaCl, CaCl
2
 & MgO etc. (Ionic Bond)
Sharing of electrons between atoms.
(a) Sharing of equal number of electron between two atoms.
Ex. Cl
2
, N
2
, O
2
 etc., (Covalent bond)
(b) Sharing of electron pair given by only one atom
Ex. [NH
3
 ?? H
+
] & NH
3
 ?? BF
3
 (Co-ordinate Bond)
EXCEPTIONS OF OCTET RULE
1 . Transition 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)
— —
BeF
2 —
BF
3 — —
AlCl
3 —
B Cl
3
(4e) (6e) (6e) (6e)
4 . Expansion of Octet (due to empty d-orbitals)
PCl
5
SF
6
ClF
3
ICI
5
IF
7
? ? ? ? ?
(10e) (12e) (10e) (12e) (14e)
5 . Odd electron species
Ex. NO, NO
2
, ClO
2
 etc.
N
O
O
    ,      
N 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 XeF
2
, XeF
6
 & KrF
2
 etc., have been actually prepared.
CLASSIFICATION 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
H m olecule
2
          
?
?
?
?
? ? ? ?
?
?
?
?
?
? ? ? ?
?
?
? O = O
O O
×
×
×
×
×
×
×
×
       
?
?
?
?
?
? ?
?
?
?
? ?
? N  N ?
N N
×
×
×
×
× ×
(b) The shared pair of electrons should have opposite spins, and are localised between two atoms
concerned.
F
|
|
|
.
.
.
B F
F
(6 Electrons)
×
×
×
F
F
F F
F
F S
(12e)
(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     
? ? ? ?
?NH
3
 ??BF
3
         ? ? ? ? ? ? ? ? ? ?
        H      F
BF
3
 is electron defficient compound.
Metal co-ordinate compounds - [Cu(NH
3
)
4
]
+2
?
H N
3
H N
3
NH
3
NH
3
?
?
?
?
?
?
?
?
Cu
+2
Ex. NH ;      HN  +  H H – N –H
4 3
+ + 
? ?? ? ? ?
H
H
(Lowry-Bronsted acid)
       (e acceptor)
–
+
HO ;          
3
+
H     
+
? ? ? ?
H H
H H
O O O +
? ?
?
?
? ?
?
?
? ? 
H
+
N
2
O  ; N ? 
?
? N ??O
? Note : Compounds in which Ionic, covalent and co-ordinate bonds are present, are as follows -
NH
4
Cl,    CuSO
4
.5H
2
O,    K
4
[Fe(CN)
6
],    KNC, KNO
3
, etc.
Page 4


CHEMICAL BONDING
INTRODUCTION
(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 . Tendency to acquire minimum energy :
(a) When two atoms approaches to each other-
Nucleus of one atom attracts the electron of another atom. 
Potential Energy diagram
(b) Two nuclei and electron of both the atoms repells each other.
(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 (s
2
p
6
) configuration in their outermost orbit.
The octet may be complete in following manner:
Complete transfer of electrons from one atom to another.
Ex. NaCl, CaCl
2
 & MgO etc. (Ionic Bond)
Sharing of electrons between atoms.
(a) Sharing of equal number of electron between two atoms.
Ex. Cl
2
, N
2
, O
2
 etc., (Covalent bond)
(b) Sharing of electron pair given by only one atom
Ex. [NH
3
 ?? H
+
] & NH
3
 ?? BF
3
 (Co-ordinate Bond)
EXCEPTIONS OF OCTET RULE
1 . Transition 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)
— —
BeF
2 —
BF
3 — —
AlCl
3 —
B Cl
3
(4e) (6e) (6e) (6e)
4 . Expansion of Octet (due to empty d-orbitals)
PCl
5
SF
6
ClF
3
ICI
5
IF
7
? ? ? ? ?
(10e) (12e) (10e) (12e) (14e)
5 . Odd electron species
Ex. NO, NO
2
, ClO
2
 etc.
N
O
O
    ,      
N 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 XeF
2
, XeF
6
 & KrF
2
 etc., have been actually prepared.
CLASSIFICATION 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
H m olecule
2
          
?
?
?
?
? ? ? ?
?
?
?
?
?
? ? ? ?
?
?
? O = O
O O
×
×
×
×
×
×
×
×
       
?
?
?
?
?
? ?
?
?
?
? ?
? N  N ?
N N
×
×
×
×
× ×
(b) The shared pair of electrons should have opposite spins, and are localised between two atoms
concerned.
F
|
|
|
.
.
.
B F
F
(6 Electrons)
×
×
×
F
F
F F
F
F S
(12e)
(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     
? ? ? ?
?NH
3
 ??BF
3
         ? ? ? ? ? ? ? ? ? ?
        H      F
BF
3
 is electron defficient compound.
Metal co-ordinate compounds - [Cu(NH
3
)
4
]
+2
?
H N
3
H N
3
NH
3
NH
3
?
?
?
?
?
?
?
?
Cu
+2
Ex. NH ;      HN  +  H H – N –H
4 3
+ + 
? ?? ? ? ?
H
H
(Lowry-Bronsted acid)
       (e acceptor)
–
+
HO ;          
3
+
H     
+
? ? ? ?
H H
H H
O O O +
? ?
?
?
? ?
?
?
? ? 
H
+
N
2
O  ; N ? 
?
? N ??O
? Note : Compounds in which Ionic, covalent and co-ordinate bonds are present, are as follows -
NH
4
Cl,    CuSO
4
.5H
2
O,    K
4
[Fe(CN)
6
],    KNC, KNO
3
, etc.
WAVE MECHANICAL CONCEPT OF CO-VALENT BONDING –
(a) One orbital can accomodate at the most 2 electrons with opposite spins
(b) Half filled orbital or unpaired electron orbital accepts one electron from another atom, to complete its orbital.
(c) Tendency to complete orbital or to pair the electron is an condition of covalent bond.
Completion of octet is not the essential condition of covalent bond.
(d) If the outermost orbit has empty orbitals then covalent bonds are formed in  excited state.
Variable valency in covalent bonds :
(i) Variable valencies are shown by those elements which have empty orbitals in outermost shell.
(ii) Lone pair electrons get excited in the subshell of the same shell to form the maximum number of
unpaired electrons. Maximum covalency is shown in excited state.
(iii) The energy required for excitation of electrons is called promotion energy.
(iv) Promotion rule – Excitation of electrons in the same orbit.
Ex.
(I) Phosphorus ? ? Ground state
Covalency 3 (PCl
3
)
   3s     3p
Phosphorus ? ? Excited state
Covalency – 5 (PCl
5
)
   3s        3p            3d
(II) Sulphur ? ? Ground state.
   3s       3p             3d Covalency - 2 (SF
2
)
Sulphur ? ? Excited state
I
st 
excited state ? Covalency - 4 (SF
4
)
  3s        3p            3d
2
nd 
excited state ? Covalency - 6 (SF
6
)
          3s 3 p 3d    So variable covalency of S is 2, 4, & 6.
(III) Iodine has three lone pair of electrons
(Ground state) ?
5s       5p    5d
So it shows three excited states – Maximum number of unpaired electrons = 7
Variable Valencies are 1, 3, 5, 7
To explain the formation of covalent bond two theories based on quantum mechanics have been proposed.
 ?   Valence bond theory (VBT) ?  Molecular orbital theory (MOT)
? VALENCE BOND THEORY (VBT)
(A) Overlapping theory (B) Hybridisation theory
Page 5


CHEMICAL BONDING
INTRODUCTION
(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 . Tendency to acquire minimum energy :
(a) When two atoms approaches to each other-
Nucleus of one atom attracts the electron of another atom. 
Potential Energy diagram
(b) Two nuclei and electron of both the atoms repells each other.
(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 (s
2
p
6
) configuration in their outermost orbit.
The octet may be complete in following manner:
Complete transfer of electrons from one atom to another.
Ex. NaCl, CaCl
2
 & MgO etc. (Ionic Bond)
Sharing of electrons between atoms.
(a) Sharing of equal number of electron between two atoms.
Ex. Cl
2
, N
2
, O
2
 etc., (Covalent bond)
(b) Sharing of electron pair given by only one atom
Ex. [NH
3
 ?? H
+
] & NH
3
 ?? BF
3
 (Co-ordinate Bond)
EXCEPTIONS OF OCTET RULE
1 . Transition 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)
— —
BeF
2 —
BF
3 — —
AlCl
3 —
B Cl
3
(4e) (6e) (6e) (6e)
4 . Expansion of Octet (due to empty d-orbitals)
PCl
5
SF
6
ClF
3
ICI
5
IF
7
? ? ? ? ?
(10e) (12e) (10e) (12e) (14e)
5 . Odd electron species
Ex. NO, NO
2
, ClO
2
 etc.
N
O
O
    ,      
N 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 XeF
2
, XeF
6
 & KrF
2
 etc., have been actually prepared.
CLASSIFICATION 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
H m olecule
2
          
?
?
?
?
? ? ? ?
?
?
?
?
?
? ? ? ?
?
?
? O = O
O O
×
×
×
×
×
×
×
×
       
?
?
?
?
?
? ?
?
?
?
? ?
? N  N ?
N N
×
×
×
×
× ×
(b) The shared pair of electrons should have opposite spins, and are localised between two atoms
concerned.
F
|
|
|
.
.
.
B F
F
(6 Electrons)
×
×
×
F
F
F F
F
F S
(12e)
(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     
? ? ? ?
?NH
3
 ??BF
3
         ? ? ? ? ? ? ? ? ? ?
        H      F
BF
3
 is electron defficient compound.
Metal co-ordinate compounds - [Cu(NH
3
)
4
]
+2
?
H N
3
H N
3
NH
3
NH
3
?
?
?
?
?
?
?
?
Cu
+2
Ex. NH ;      HN  +  H H – N –H
4 3
+ + 
? ?? ? ? ?
H
H
(Lowry-Bronsted acid)
       (e acceptor)
–
+
HO ;          
3
+
H     
+
? ? ? ?
H H
H H
O O O +
? ?
?
?
? ?
?
?
? ? 
H
+
N
2
O  ; N ? 
?
? N ??O
? Note : Compounds in which Ionic, covalent and co-ordinate bonds are present, are as follows -
NH
4
Cl,    CuSO
4
.5H
2
O,    K
4
[Fe(CN)
6
],    KNC, KNO
3
, etc.
WAVE MECHANICAL CONCEPT OF CO-VALENT BONDING –
(a) One orbital can accomodate at the most 2 electrons with opposite spins
(b) Half filled orbital or unpaired electron orbital accepts one electron from another atom, to complete its orbital.
(c) Tendency to complete orbital or to pair the electron is an condition of covalent bond.
Completion of octet is not the essential condition of covalent bond.
(d) If the outermost orbit has empty orbitals then covalent bonds are formed in  excited state.
Variable valency in covalent bonds :
(i) Variable valencies are shown by those elements which have empty orbitals in outermost shell.
(ii) Lone pair electrons get excited in the subshell of the same shell to form the maximum number of
unpaired electrons. Maximum covalency is shown in excited state.
(iii) The energy required for excitation of electrons is called promotion energy.
(iv) Promotion rule – Excitation of electrons in the same orbit.
Ex.
(I) Phosphorus ? ? Ground state
Covalency 3 (PCl
3
)
   3s     3p
Phosphorus ? ? Excited state
Covalency – 5 (PCl
5
)
   3s        3p            3d
(II) Sulphur ? ? Ground state.
   3s       3p             3d Covalency - 2 (SF
2
)
Sulphur ? ? Excited state
I
st 
excited state ? Covalency - 4 (SF
4
)
  3s        3p            3d
2
nd 
excited state ? Covalency - 6 (SF
6
)
          3s 3 p 3d    So variable covalency of S is 2, 4, & 6.
(III) Iodine has three lone pair of electrons
(Ground state) ?
5s       5p    5d
So it shows three excited states – Maximum number of unpaired electrons = 7
Variable Valencies are 1, 3, 5, 7
To explain the formation of covalent bond two theories based on quantum mechanics have been proposed.
 ?   Valence bond theory (VBT) ?  Molecular orbital theory (MOT)
? VALENCE BOND THEORY (VBT)
(A) Overlapping theory (B) Hybridisation theory
(A) OVERLAPPING THEORY :
(1) It was presented by Heitler & London to explain how a covalent bond is formed.
(2) The main points of theory are –
(a) To  form a covalent bond overlapping occurs between half filled valence shell orbitals of the two atoms.
(b) Resulting bond acquires a pair of electrons with opposite spins to get stability.
(c) Orbitals come closer to each other from the direction in which there is maximum overlapping
(d) So covalent bond has directional character.
(e) Extent of overlapping ? ? strength of chemical bond.
(f) Extent of overlapping depends on two factors.
(i) Nature of orbitals – p, d and f are directional orbitals ? ? more overlapping
s-orbital ? ? non directional – less overlapping
(ii) Nature of overlapping –
Co-axial overlapping - extent of overlapping more.
Collateral overlapping - extent of overlapping less
Order of strength of Co - axial overlapping – p - p > s - p > s - s
• •
• •
• •
p - p                p–s         s–s
(g) As the value of n increases, bond strength decreases.
    1s - 2p > 2s - 2p > 3s - 3p
(h) If n is same           2p - 2p > 2s - 2p > 2s - 2s
(i) Electron which is already paired in valency shell can enter into bond formation, if they can be unpaired
first and shifted to vacant orbitals of slightly higher energy of the same energy shell.
(j) This point can explain the trivalency of boron, tetravalency of carbon, pentavalency of phosphorus etc.
(k) Three types of bonds are fomed on account of overlapping.
(1) Sigma ( ?) bond (2) Pi ( ?) bond (3) delta ( ?) bond
1 . Sigma ( ?) bond :
When orbitals overlaps along their inter nuclear axis, ?-bond is formed Ex. The bond formed by overlapping of
s - s, s - p, p - p (axial), sp
3
 – s, sp
2
 – s, sp
3
 – sp
3
, sp
3
 – sp
2
 & sp – sp atomic orbitals.
 
? ? ? ? ? ? ? ?
+
Sigm a bond
S – S overlapping