Why do Ionic Bonds Form?
Ionic bonds are formed when one of the two atoms that are reacting has excess electrons and transfer the electrons to the atom that is deficient in electrons. During the formation of the ionic bond, one of the reacting atoms will donate electrons and form positive ion. The other atom forms negative ion by accepting the electrons. The ionic bond can also be defined as a bond that forms between a metal ion and a non metal ion. The metal ion is a positive ion with the tendency to donate electrons while the non metal ion is a negative ion with the tendency to accept the electrons from the metal ion. The ionic bond is formed due to electrostatic attraction between the two oppositely charged ions. The two atoms exchange the electrons with each other such that both of them attain stable electronic configuration.
For example, in the case of table salt sodium chloride or NaCl, sodium atom loses one electron and form cation or positive ion. The chlorine atom gains one electron and form negative ion. Both of them obey the octet rule and try to get the electronic number equivalent to the electronic number of nearest noble gas. The removal of electrons from the atoms makes the ion to have high energy and similarly acceptance of electrons by the ions will also adds energy to the ion. But attraction of oppositely charged ions to form a bond will lower the individual energies of the ions.
Ionic bonding is supposed to complete its formation when the resultant energy change is favorable. The energy of the atoms after ionic bond formation should be less than the individual energies of the ions in order to form an ionic bond. If there is quite a big difference in energy change then the ionic bond formed will be stronger. In short, strength of ionic bonding depends on the degree of difference in the electro negativities of the two reacting atoms.