Ionic Bonding

Thursday, November 19, 2015

Ionic bonds are formed when an electropositive element react with an electronegative element. The molecules formed due to ionic bond are polar in nature. Some examples are Sodium Chloride & Magnesium Chloride.

Properties of Ionic Molecules 

  1. Hardness: - Since nature of bonding in ionic compound is electropositive and extend in all directions, hence molecules formed by ionic bonding are very hard.
  2. Melting and Boiling Point: - Because ionic bond involves electrostatic force of attraction hence the molecule formed by ionic bond is strong and omnidirectional it involves the breaking of entire lattice, which requires higher energy. Hence melting and boiling point are high.
  3. Conductivity: - They have low electrical conductivity in solid state but are good conductors in molten state because free movement ions in molten state.
  4. Solubility: - Ionic compounds are often soluble in polar solvents such as water. This is due to polar bonding in ionic molecules.
  5. Reactivity: - Due to involvement of ions chemical reactivity of ionic compounds are generally higher.

Structure of Ionic Molecules

Ionic compounds are generally crystalline structure and it is found in solid state at room temperature. Let’s look at the following picture:-
Structure of Ionic Bonds

Among the above three structures of a molecule having coordination number 3, ‘A’ is most stable as the ions involved are closely packed and the attractive forces very well counter the repulsive forces as the size of cation increases more number of anions are required to stabilize the ionic solid. So, the coordination number also increases therefore radius ratio is an important parameter to determine the structure of the ionic solid. But it suffers various assumptions like molecule is a perfect sphere but in real that is not possible.
Radius Ratio

A molecule with a lower radius ratio value indicates greater stability.
Nowadays a technique known as X-Ray Crystallography is used to determine the complete structure of ionic solid.

Energy Consideration

Lattice Energy:-

The amount of heat released when one mole of lattice is formed from its constituent gaseous ions is known as lattice energy.
Amount of heat absorbed when one mole of lattice is broken to its constituent gaseous ions is known as lattice energy.
Lattice Energy Determination

Born-Haber Cycle

Lattice energy can be determined experimentally by Born Haber cycle, which is based on the Hess’s law of constant heat summation.
Hess’s Law- It states that the energy involved in a particular reaction is same irrespective of the reaction proceeds in one step or via several steps.
Let’s we consider a general reaction of an ionic compound formation:-                          
Born Haber Cycle
From Hess’s law
Hess Law of constant heat summation

Born Lande’s Equation

According to Born Lande’s equation minimum lattice energy is given as follows: -
Born Lande equation
A = Med lung Constant (sum of all geometric interactions)
Z+ & Z- = charges on cation and anion respectively
N0 = Avogadro number
r = Inter ionic distance
n = Compressibility Factor (Born Constant)

Born Lande’s equation states that lattice energy is directly proportional to product of the charges of cation and anion present in the ionic compound. While it is inversely proportional to inter ionic distances. For example you can see the change in lattice energy due to change in product of charges and also due to change in inter ionic distances in following examples.
Ionic molecules comparison

From table ‘A’ we can see that where inter ionic distance in Lithium Fluoride and Magnesium Oxide are almost same while product of charges is 4 times more in the case of magnesium oxide than Lithium Fluoride hence lattice energy is energy is also approximately 4 fold in the case of MgO. This follows Born Lande’s equation which states that lattice energy is directly proportional to product of charges.
From table ‘B’ we can see that intrinsic distance is almost four times more in CsI than LiF and hence lattice energy is half in CsI than LiF. This also proves Born Lande’s equation true which states that lattice energy is inversely proportional to inter ionic distances.