Electrovalent or Ionic bond:

When a bond is formed by complete transference of electrons from one atom to another so as to complete their outermost orbitals by acquiring 8 electrons or 2 electrons in hydrogen , lithium etc. and hence acquires the stable nearest noble gas configuration ,the bond that formed is called ionic bond.

For e.g.      Na     →       Na⁺ + e^–

                   (2,8,1)              (2,8)

                   Cl  +  e^–  →       Cl^–

                     (2,8,7)              (2,8,8)

The force between the two types of ions is electrostatic force of attraction.

Note:

Generally this type of bond forms between the elements having big difference in their Electronegativities.The number of electrons lost or gained during the formation of an electrovalent linkage is termed as the electrovalency of the element.

Co-ordination number:

Co-ordination number of an ion may be defined as the number of oppositively charged ions present as the nearest neighbours around that ion in an ionic crystal.

Factors governing the formation of Ionic bonds:

1. Ionization Enthalpy:

Ionization enthalpy of any element is the amount of energy required to remove an electron from the outermost shell of an isolated gaseous atom in gaseous phase.It is clear that lesser the ionization enthalpy, easier will be the removal of an electron i.e. formation of a positive ion & hence greater the chances of formation of an ionic bond.

2. Electron gain Enthalpy (Electron affinity) :

Electron gain enthalpy of an element is the enthalpy change that takes place when an extra electron is added to an isolated atom in the gaseous phase. Higher is the electron affinity, more is the energy released and stable will be the negative ion produced, consequently, the probability of formation of ionic bond will be enhanced.

3. Lattice Enthalpy :

The energy released when one mole of the ionic compound is formed from respective ions is called Lattice Enthalpy. The higher the value of lattice enthalpy of the resulting ionic compound, the greater will be the ease of its formation.

force of attraction 

Hence the value of Lattice Enthalpy depends upon the following two factors:

1. Charge on the ions :

More the charge on the ions, greater is the force of attraction and hence larger is the amount of energy released.

2. Size of the ions :

If the size of the ions is large, inter nuclear distance will be more and force of attraction will be less, while in case of small ions, inter nuclear distance is less and so force of attraction is greater.

Net effect:

If Lattice enthalpy + electron gain enthalpy > ionization enthalpy, the net effect will be the release of energy & hence an ionic bond is formed.

Fajan’s rule :

When two oppositely charged ions come close, the positive ion tends to distorts the electron cloud of negative ion towards itself. Consequently , the electron cloud of negative ion gets polarised and electron density is pulled in between the nuclei of the two atoms.

We can say , the ionic bond does not remain 100% ionic but develops some covalent character.

(a) The power of cations(ion having positive charge) to cause distortion in the electron cloud of negative ion is referred to as its Polarising Power.

(b) The power of anion to undergo distortion is called its Polarisability.

The extent of covalent character in ionic bond depends on the polarising power of cation and polarisability of anion which are decided on the set of rules called Fajan rules:

1. The smaller the size of the cation the greater the covalent character of an ionic bond.

For example, Li⁺ is smaller than K⁺ ion. Therefore, LiCl has more covalent character than KCl.

2. Bigger the size of the anion , the greater the covalent character of an ionic bond.

Lil is more covalent than LiF. Similarly AIF₃ is ionic but AlCl₃ is covalent in nature because ionic radius of I^– is greater than Cl^– and that of Cl^– is greater than F^–.

3. More the charge on the cation, the more will be the covalent character of the ionic bond.

4. For two cations of the similar size and charge, the one, with electronic configuration (n-1)d¹⁰ns⁰, i.e., Pseudo noble gas configuration, is more polarising than the one with a noble gas configuration ns² np⁶, typical of alkali and alkaline earth metal cations. For example, CuCl is more covalent than NaCl because polarising power of Cu⁺ ion which has Pseudo noble gas i.e., 18 valence electrons configuration is more than Na⁺

Note: Small cation, large anion and high charge on both favour covalency.

Types of Crystals:

1. Face centred cubic:

A crystal having the lattice point at the face centre in addition to the lattice point at every corner is called FCC crystal. Considering an atom at the face centre as origin, it will be found that this face is common to two cubes & there are 12 points surrounding it situated at a distance which is equal to half of the face diagonal of the unit cell.

2. Body centred cubic:

A crystal having the lattice point at the body centre in addition to the lattice point at every corner is called BCC crystal.Here the central atom is surrounding by eight equidistant atoms & hence the coordination number.

Radius Ratio :

Radius Ratio = Radius of cation

                               Radius of anion

Calculation of Lattice Energy:

The lattice energy of an ionic solid is determined experimentally by a process known as Born Haber cycle. This cycle is based on hess’s law i.e. the formation of an ionic crystal may occur either by direct combination of the elements or by an alternate process in which the following steps are involved.

1.The reactants are converted into gaseous state.

2. The gaseous atoms are converted into ions.

3. The gaseous ions are combined to form an ionic compound.

For example, the formation of KF can occur either directly or in steps.

Direct Combination:

K(s) + ½ F₂ (g)   →   KF (s)

Heat of formation = -562 kjmol^-1

In Steps :

(a) Conversion of solid potassium into gaseous state

K(s)       →    K(g)

Heat of atomisation = 89.6 kjmol^-1

(b) Formation of a cation

K(g)           →            K⁺(g) + e^–

Ionisation Enthalpy = 419 kjmol^-1

(c) Conversion of molecular fluorine into gaseous atomic fluorine

½ F₂ (g)      →            F (g)

½ ( Heat of atomisation) = 79.1 kjmol^-1

(d) Formation of anion

F(g) + e^–   →              F^–(g)

Electron Affinity = -332.6 kjmol^-1

(e) Combination of K⁺(g) & F^–(g) to form KF(s)

 K⁺(g) +  F^–(g)      →              KF(s)

Lattice Energy = -U kjmol^-1

On the basis of Hess’s law

-562.6 = 89.6 + 419 +79.1 -332.6 –U

U = 817.7 kjmol^-1

Lattice Energy of KF = – 817.7 kjmol^-1