The electrostatic force of attraction which holds the oppositely charged ions together is known as ionic bond or electrovalent bond.
We can understand the formation of ionic bond by considering the examples of sodium chloride Na (1s2 2s2 2p6 3s1) has only one electron in the valence shell. It can acquire the stable electronic configuration of neon (152 252 2p6) by losing the electron. On the other hand chlorine atom (1s2 2s2 2p6 3s2 3p5) has seven electron in its valence shell and it needs only one electron to complete its octet state. Thus both the atoms can complete their octet state if sodium atom gives one electron to chlorine atom. This tendency is responsible for bonding between sodium and chlorine atoms. Therefore sodium gives one electron and becomes positive charged Na+ ion, while chlorine takes up the electron and becomes negatively charged, Cl– ion.
11Na (1s2 2s2 2p5 3s1) —> Na+ (1s2 252 2p6) + e
17Cl(1s2 2s2 2p6 3s2 3p5) + e —> Cl– (1s2 252 2p6 3s2 3p6)
Na+ + Cl– —> NaCl
These two ions are then holding together by electrostatic force of attraction which constitutes the ionic bond.
General characteristics of electrovalent compound:
- All ionic compounds are usually crystalline solids and are composed of ions even in the solid state.
- Ionic solids have high melting points and boiling points due to the presence of strong attractive forces between the oppositely charged ions.
- Ionic compounds have low volatility, high density and high stability.
- These are highly soluble in polar solvents (such as water) having high dielectric constant but insoluble in organic solvents (such as benzene, alcohol, ether, etc.). However, some ionic compounds like BaSO4, PbSO4, AgCl, AgBr, AgI, etc are insoluble in water.
- In molten state or in solution in polar solvents, ionic compounds are good conductors of electricity.
- In solution, ionic compounds undergo ionic reactions which are very fast.
- Crystals of certain ionic compounds have similar arrangement of atoms as well as geometry. Such crystalline compounds are called isomorphs and the phenomenon is called isomorphism. For example, ZnSO4.7H2O and FeSO2.7H2O are isomorphs. Also all alums show isomorphism.
- Ionic bonds are non-directional and due to the non-directional nature of ionic bonds, ionic compounds do not show isomorphism.
- Ionic compounds have high densities. This is so because the electrostatic force existing between the oppositely charged ions brings these ions close to each other. This results in decrease in volume of the crystal and hence increases in density.