On the right side of the periodic table, elements exhibit reluctance to lose their valence electrons due to their high electron affinity. Therefore, the energy required to remove an electron is enormous as compared to that needed to gain an electron. Due to the need for electron pairing the atoms, there is need to share their unpaired electrons to form stable entities, molecules.
Each atom has to donate a single unpaired electron for a true covalent bond to be formed. However, in some instances one atom is required to donate both electrons. In this case, the bond is referred to as a dative covalent bond. Bonding can occur between atoms of the same element or it can occur between dissimilar atoms. In the latter case, charge distribution within the molecule is skewed with the more electronegative atom possessing a partial negative charge and the less electronegative one having a partial positive charge.
For example, two chlorine atoms combine covalently to form chlorine molecules, Cl+Cl Cl2.This is the stable independent unit of chlorine gas. Similarly, nitrogen combines with hydrogen in a ratio of 1:3 to form ammonia, NH3.
