The reactivity of the elements is determined by the tendency to have a completely filled valence shell. The electron dot structure of Na (sodium) and Cl (chlorine) atoms are shown in the figure below.

Electron dot structure of Na and Cl

To attain completely filled valence shell, Na has to lose the outermost electron, and Cl needs one electron to complete the valence shell.

By losing an electron, Na becomes Na⁺ (cation) and this electron is accepted by Cl to form Cl^- (anion). The oppositely charged ions attract each other and form NaCl. They are held together by strong electrostatic forces of attraction. The bond existing between them is called an ionic bond.

Figure 1: Formation of sodium chloride

Since the inter-ionic forces are very strong, it requires a considerably large amount of energy to break the bond. Hence, ionic compounds have high melting point.