ü Types of
chemical Bonds: The chemical bonds
are classified into following types
· Ionic
bond
· Covalent
bond
· Dative
bond or Co-ordinate bond
· Hydrogen
bond
· Metallic
bond
· Ionic
bond: A bond formed by the transfer
of electron between 2 atoms is called ionic or electrovalent bond. Or, it is the force of attraction
between two oppositely charged ions. These ions are held together by
electrostatic force of attraction. The number of electrons lost or gained is known
as the electro-valency.
Na → Na+ +e-
2,8,1 2,8
Cl + e- → Cl-
2,8,7 2,8,8
Na+ + Cl- → NaCl
Let us
take another example of MgCl2 (Magnesium Chloride): 
Ø
Energy changes during the formation of an Ionic Bond: An ionic bond formation involves the
following steps:
1) The
formation of a positive ion (cation) from the electropositive atom. The amount of energy required
for this process is called ionisation enthalpy
(∆iH).
M(g) + ∆iH → M+(g)
+ e-
2) Formation of negative ion from the
electronegative atom. The energy involved in this process is called
electron gain enthalpy (∆egH).
X(g) + e- → X-(g)
+ ∆egH
3) The packing of the cation and anion to
form an ionic compound. The energy change in this process is called lattice
enthalpy (∆latticeH)
M+ + X- → MX(s)
+ ∆latticeH
Ø Factors
favouring formation of ionic bond:
·
Low ionization enthalpy (IE)
·
More negative electron gain enthalpy (EGE)
·
High lattice enthalpy (LE) If LE + EGE > IE, ionic bond is formed.
Ø Properties
of ionic compounds:
·
Non-directional nature of ionic bond: These compounds do not exist as
individual molecules. Instead they are often hard solids, always made up
of ions held together by strong
electrostatic forces of attraction between ions with opposite charge.
·
Electrical conductivity: When added to water, the ions separates.
The solution can thus conduct electricity. Similarly, these compounds conduct
electricity in the fused state also.
·
Melting and boiling points: These
compounds generally have high melting points. This is because a lot of thermal energy is required to
break down the inter-ionic forces and form a liquid. Similarly, they
possess high boiling points.
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