IONIC OR ELECTROVALENT BOND

IONIC OR ELECTROVALENT  BONDS

a) Ionic bond is formed by transfer of electron from electropositive to electronegative atoms. Atom losing electrons becomes cation while the atom gaining electron becomes anion. The electrostatic force of attraction holding the oppositely charged ions are called electrovalent bond or ionic bonds.

b) It was proposed by KOSSEL.

c) Ionic bond is non-directional.

d) The number of electrons which an atom gains or loses while forming ionic bond is called electro-valency.

The necessary condition for stable ionic bonding are :

a) Ionisation Enthalpy of cation forming element should be low.

b) EA of anion forming element should be high.

c) Lattice energy should be high.

Lattice energy :

Lattice energy is the amount of energy released when one mole of ionic solid is formed from its constituents ions in its gaseous state.

a) Lattice energy depends on the size of the ions and magnitude of the charge on them.

b) In general, for the same anion the magnitude of the lattice energy decreases with the increase in the size of the cation.

Ex. MgO > CaO > SrO > BaO

c) For the same cation , the magnitude of the lattice energy decreases with the increase in size of the anion.

Ex. NaF > NaCl > NaBr > NaI.

d) Lattice energy increases from univalent ionic solid (NaCl) to uni-bivalent (MgCl₂) and then bivalent (MgO) ionic solids.

e) Larger the magnitude of the lattice energy, greater will be the stability of the ionic solid.

f) Ionic solids have higher value of lattice energies have higher melting and boiling points.

g) Ionic solids have high  values of lattice enthalpy have low solubility in water.

  

  CHARCTERISTICS OF IONIC COMPOUNDS :

a) They have high melting and boiling points due to strong attractive forces between the ions.

b) Due to fixed position occupied by the ions in solid state, they do not conduct electricity. However, they      becomes good conductors in their molten or aqueous state.

c) They are soluble in polar solvent with hi dielectric constant such as water. On dissolution in water ionic compounds undergo hydration and the energy released in this process is called hydration energy. For dissolution of ionic compounds in water         ΔH_hydration > ΔH_lattice.

d) In aqueous solutions, the ionic compounds forms free ions which readily undergo ionic reaction.

e) Ionic solids are brittle in nature. Stress on ionic crystals brings similarly charged ions close to one another which cause repulsive forces and results in breaking.

COVALENT CHARACTER IN IONIC COMPOUNDS:

FAJAN’S RULE

In the ionic compounds the cation tends to polarize the electron cloud of the anion by pulling electron density towards itself. This cause development of covalent character in ionic bonds because the electron density get localized in between the nuclei.

a) The tendency of cation to bring about the polarization of anion is expressed as its polarizing power.

b)  The ability anion to undergo polarization is called its polarisability.

The polarizing power and polarisability of cation and anion respectively are  decided on the basis of Fajan’s rule as given below:

a) Smaller the cation higher is its polarizing power.

b) Cations with pseudo noble gas configuration (ns² np⁶ nd¹⁰) have relatively high polarizing power than those of noble gas configuration (ns² np⁶).

c) Larger the size of anion higher is its polarisability.

  Thus, smaller the cation and larger the anion, higher will be the covalent character in ionic bonds.

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