Jee notes and pyq

Chemical Bonding 95



8 Chemical Bonding
QUICK LOOK Covalent Bond: Covalent bond is formed by mutual sharing of
electrons so as to complete their octet or duplet in case of H, Li
A molecule is formed if it is more stable and has lower energy than and Be. Depending upon whether one, two and three electrons
the individual atoms. Normally only electrons in the outermost are shared by each atom, single, double and triple bonds are
shell of an atom are involved in bond formation and in this process respectively formed. The number of electrons contributed by
each atom attains a stable electronic configuration of inert gas. each atom for sharing is called its Covalency.
Atoms may attain stable electronic configuration in three different When two half-filled atomic orbitals overlap along their
ways by loosing or gaining electrons by sharing electrons. The internuclear axis, the bond formed is called the sigma bond (or
attractive forces which hold various constituents (atoms, ions, etc)  - bond). It may be formed by the overlap of two s-orbitals, two
together in different chemical species are called chemical bonds.
p-orbitals, one s- and one p-orbital. Accordingly these bonds are
Elements may be divided into three classes.
designated as s-s, p-p and s-p sigma bonds. On the other hand,
 Electropositive elements, whose atoms give up one or more
electrons easily, they have low ionisation potentials. covalent bond formed by sideways or lateral overlap of p-orbitals
 Electronegative elements, which can gain electrons. They is called pi-bond (or  - bond). It may be noted that
have higher value of electronegativity.  All single bonds are  - bonds.
 Elements which have little tendency to loose or gain  Multiple bonds contain one - bonds and the rest are
electrons.  - bonds.
Three different types of bond may be formed depending on  A  - bond is never formed alone. First,  - bonds is formed
electropositive or electronegative character of atoms involved. and then the formation of the  - bond takes place.
Electropositive element + Electronegative element = Ionic bond  A sigma bond is always stronger than pi-bond because the
(electrovalent bond)
extent of overlapping of atomic orbitals along internuclear
Electronegative element + Electronegative element = Covalent bond
axis is greater than sideways overlapping.
or less Electropositive + Electronegative element = Covalent bond
Electropositive + Electropositive element = Metallic bond. Coordinate or Dative Bon: This type of bond formation
Fajan Rules: In Ionic bond, some covalent character is occurs by one sided sharing of electrons, i.e., one atom donates
introduced because of the tendency of the cation to polarise the a pair of electrons while the other simply shares it so as to
anion. In fact cation attracts the electron cloud of the anion and complete its octet. The atom that donates a pair of electrons is
pulls electron density between the two nuclei. called the donor while the other which accepts these electrons
is called the acceptor. The coordinate bond is usually
represented by an arrow pointing from the donor towards the
accept

Cation  
Anion Polarised electron H H
Figure: 8.1 cloud of anion |  | 

H
Example: H — N:  or  H — N— H 
According to Fajan rules, the magnitude of covalent character |  | 
H  H 
in the ionic bond depends upon the extent of polarisation  
caused by cation. In general, Coordinate bond is also present in SO 2 ,SO 3,O 3, H O
3 , NO 
3
 Smaller the size of cation, large is its polarising power.
or HNO3 etc.
 Among two cations of similar size, the polarising power of
This coordinate bond has some polar character, it is also called
cation with noble gas configuration (ns 2np nd 6 10
) is larger
dative or semi-polar bond.
than cation with noble gas configuration (ns 2np 6) e.g., In terms of VB theory, a coordinate bond is formed by overlap
Polarising power of Ag  is more than K  of a fully filled orbital containing a lone pair of electrons with
 Larger the anion, more will be its polarisability an empty orbital of another atom.

, 96 JEE-Main and Advanced Chapter-wise Solved Papers: Chemistry
Examples of molecules in which all the three types of bonds, N2O 3 / O  N  N  O

NO 3 / O N  O SO3 / O  S  O

 O
i.e., ionic, covalent and coordinate bonds are present: O O
CuSO 4. 5H 2O, NH Cl,
4 K  
Fe
4 CN 
6,  Cu NH SO , etc.
3 4 4
HClO3 / SOCl 2 / O  S  Cl
|
HIO3 /

H  O  Cl  O Cl H  O  I O
Besides these bonds, CuSO 4.5H O
2 also contains a H-bond.  
O O

Characteristics of coordinate compounds are: H2O 2 / H  O  O  H Cl
|
SO2Cl2 / O  S  O
Since coordinate compounds are in fact covalent compound, or H O  O
|
|
Cl
therefore, their properties are almost similar to those of H
O
covalent compounds. For example, 
HNO 2 / H  O  N  O
HClO4 / H O  Cl  O
 Like covalent compound, they are insoluble in H2O but are 
O
soluble in organic solvents.
NO 2 /  O  N  O SO 2 / O S  O O
 They usually do not conduct electricity. 
H 2SO 4 / H O  S  O H
 Their melting and boiling points are higher than those of 
O
covalent compounds but lower than those of ionic compounds.
N2O4 / O  N  N  O SO 32  / H3PO4 /
 Like covalent bonds, coordinate bonds are directional and  
  
O O O  S O O  S  O
hence these compounds also exhibit stereoisomerism 
O 
O
Table: 8.1 Bond Formula or Dash Formula of Molecules Showing Different N2O 5 / O  N  O N O
Type of Bonding  
O O
(1) Compounds having (2) Compounds having covalent bonds
(5) Compounds having electrovalent bonds only
electrovalent bonds only only 
Molecular formula/Dash H2O/  H 
HCl/H – Cl  | 
formula H–O–H NH4Cl /  H  N  H  Cl
|
NaCl / Na+ Cl– H2S / H – S – H HCN/  
 H 
H–CN
MgCl2 / Cl– Mg++ Cl– C2H2 / H – C  C – H  CN 
CaCl2 / Cl– Ca++ Cl– H H H  
| | | N  C CN
NH3 / H  N C2H 6 /H —C—C— H K4[ Fe(CN)6 ] / 4k  Fe 
| | |  
H H H N  C CN
 CN 
MgO/Mg++ O– H Cl  
| |
CH4 /H —C—H PCl3 /Cl—P
| | Table: 8.2 Comparison of Ionic, Covalent and Coordinate Compounds
H Cl Property Ionic Covalent Coordinate
Na2S / Na  S   Na  H H H
| | |
C2H 4 /C C PH3 /H—P 1.Binding Between ions Between molecules In between
| | | force strong smaller (Van der
H H H Waal’s)
(coulombic)
CaH2 / H C a   H  2.mp/bp High Less than ionic In between
AlF3 / FAl3F  3. Condition conductor of Bad conductor Greater than
F electricity in covalent
(3) Compounds having electrovalent and covalent bonds fused state and
  in aqueous
NaOH / Na [O  H]
solution
 
KCN / K [C  N]
4. Solubility in High Less In between
2
  polar solvent
   (H2O)
CaCO 3 / Ca O  C O
||
 O  5. Solubility in Low High In between
non polar
(4) Compounds having covalent and coordinate bonds
solvent (ether)
CO / N2O / HNO 3 / H  O  N  O
  6. Physical generally solid Liquid and Solid, liquid
C O N N  O O state gaseous gas