A chemical bond refers to the strong electrostatic force (ie, attraction between opposite charges) binding atoms or ions together. Ionic bond is the strong non-directional electronic force of attraction between oppositely charged ions. The chemical bond between two atoms, which involves transference of one or more electrons from one atom to the other, is termed electrovalent bond or ionic bond.
Formation of ionic bond Atoms become ions by gaining or losing electrons and so electrons are transferred until the outermost electron shell is identical to that of the nearest noble gas. For example, when metal and non-metal combine, atoms of the metal donate electrons to atoms of the non-metal to form cations while atoms of the non-metal accept electrons to form anions.
The electronic structure of an ionic compound, Na+ C1- is shown below: The ions of opposite charges in ionic compounds are held together by electrostatic forces and such linkage is known as ionic bonding. The tendency to form ions is the greatest in Groups I, II, VI, and VII since atoms will form ionic compounds only if the energy evolved in the formation of the ionic crystal lattice exceeds the energy required to form the ions.
Some ionic compounds may be made up of ions containing more than two elements, eg, NH4 NO3 , (NH4 )2 SO4 . It is evident that ionic solids are not merely an assemblage of ion pairs. In sodium chloride, for example, a Na+ ion is no more or less bonded to a particular Cl- ion than it is to any of its other five nearest neighbours.
Figure 1 shows the cubic crystal lattice of sodium chloride - an ionic solid which exists as a giant lattice rather than as ion pairs.
Evidence in favour of existence of ions Many ions are colourless, but some are coloured. The colour of an ion may be deduced by observing the colour of solutions of a series of compounds.