The factors which influence the formation of an ionic bond between a metal and non-metal are discussed below:
1. Ionization Energy: The amount of energy required to remove (to infinity) the outermost electron i.e. loosely bound electron in the atom when the gas atom is isolated in free space is known as ionization energy. For the formation of an ionic bond, there must exist one atom which forms a cation by losing one or more electrons. Hence elements having electrons which are loosely bound have grater tendency to from ionic bond. Thus, metallic elements having lower ionization energy favors the formation of an ionic bond. Alkali and alkaline earth metals have greater tendency to form ionic bond because of low ionization energy associated with them.
2. Electron affinity: When an electron is added to an isolated gaseous atom, some amount of energy is released and that amount of energy released is known as Electron affinity. This can be represented as:
X + e– ————> X–
In the formation of an ionic compound the atom participating form an anion by gaining an electron (s). Formation of an anion is favored by higher electron affinity. Therefore, formation of an ionic bond will be grater in which the elements have higher electron affinity. Halogens have great tendency to form ionic compounds because of having high electron affinities.
3. Lattice Energy: When a cation and an anion get attracted because of having opposite charge, there is a release of energy and the amount of energy released is known as lattice Energy. The higher the lattice energy higher will be the tendency to form ionic bond.
The force of attraction between two oppositely charged ions in vacuum (air) is known as Coulomb’s law which is as follows:
F= [-q 1q 2 ]/[4πε 0 r 2]
Where,
q 1 and q 2 – charges of the ions respectively
r – distance between them
4πε 0 – permittivity factor
The distance between the ions will be smaller if the charges on the ions are higher. Hence the force of attraction will be greater between the ions.
Lattice Energies of various alkali metals halides (kJ mol-1)
| Halide ion | Li+ | Na+ | K+ | Rb+ | Cs+ |
| F– |
– 994.5 |
– 894.5 |
– 790.2 |
– 756.7 |
– 719.0 |
| Cl– |
– 802.6 |
– 758.7 |
– 681.4 |
– 660.6 |
– 618.7 |
| Br– |
– 760.8 |
– 714.8 |
– 656.3 |
– 631.3 |
– 593.6 |
| I– |
-710.6 |
-668.8 |
-618.7 |
-597.7 |
-564.3 |
For a given anion, Smaller is the radius of the cation, greater will be the strength of the ionic bond. Likewise for a given cation, smaller is the radius of the anion, greater will be the strength of the ionic bond.
