In this type of bond, the outer atomic shell of one atom has a vacancy which allows addition of one or more electrons. These newly added electrons potentially occupy a lower energy-state (effectively closer to more nuclear charge) than they experience in a different atom. Thus, one nucleus offers a more tightly-bound position to an electron than does another nucleus, with the result that one atom may transfer an electron to the other. This transfer causes one atom to assume a net positive charge, and the other to assume a net negative charge. The atom from which electrons are transferred gets positively charged, known as positive ion and the atom in which the electrons are transferred or added gets negatively charged and known as negative ion. The bond then results from electrostatic attraction between atoms, i.e. the atoms which are positively or negatively charged ions known as ionic bond. the ionic bond is also known as electrovalent bond. The ionic bond helps in making the atom stable by acquiring the stable electronic configurations of the nearest noble gas.
The compounds which are formed by the ionic bond are known as ionic compounds or electrovalent compounds.
Electrovalency: Electrovalency of an element is given by calculating the number of electrons transferred to or from the atom of that element in order to acquire the stable s2p6 configuration in the outer shell.
Mostly ionic bond is accompanied by decrease in energy.
For Example:
Formation of sodium chloride leads to decrease in energy which are explained as follows:
i) Na (s) + Sublimation energy —————> Na (g)
= + 108.5 kJ mol-1
ii) Cl2 (g) + Dissociation energy ——————> 2Cl (g)
= + 243.0 kJ mol-1
iii) Na (g) + Ionization energy ——————> Na+ (g) + e–
= + 495.2 kJ mol-1
iv) Cl (g) + e– ——————> Cl– (g) + Electron affinity
(= – 348.3 kJ mol-1)
v) Na+ (g) + Cl– (g) ———-> [Na+][ Cl–] + Lattice energy
(= – 758.7 kJ mol-1)
Here s means solid and g means gas.
Net change of energy for all five steps = 108.5 + ½(243.0) + 495.2 – 348.3 – 758.7
= – 381.8 kJ mol-1
Hence decrease in energy take place in the formation of sodium chloride (gas) to form sodium chloride (solid)
The net energy change is denoted by ΔH which is known as Enthalpy of formation of ionic bond. Enthalpy is the net energy change obtained by adding all the energy terms as mentioned in above example.
In ionic bond if ΔH is negative only then the ionic bond will be feasible.
