Polar Covalent Bond
In a covalent bond formed between two similar atoms, both the atoms share the bonded electrons equally. However, this is not true when the two atoms are different and where the bonded pair of electrons is not placed symmetrically with respect to each other.
Each atom has its own tendency to attract the bonded electrons towards itself. This tendency is measured by the electro negativity of the atom. An atom having higher electro negativity will have a larger tendency to attract the bonded electrons towards it. In a covalent bond formed between similar atoms, both the atoms exert an equal degree of attraction on the bonded electrons, as a result of which the bonded electrons are conventionally shown in the centre of the bond. In the case of a covalent bond involving two different atoms, the degrees of attraction are different. Hence, the electron pair lies nearer the atom having a higher degree of attraction (i.e., having higher electronegativity). Consequently, this atom acquires a partial negative charge and the other atom (with lesser electronegativity) acquires an equal partial positive charge. Such a distortion of charges gives rise to a polar covalent bond. The extent of polarity depends on the difference of electro negativities of the two involved atoms. The polarization of the bond increases its bond strength owing to the increased electrostatic attraction between the two partial charges. One of the simplest examples of a molecule exhibiting polar covalent bond is hydrogen chloride. In this molecule, chlorine being more electronegative acquires a partial negative charge whereas hydrogen acquires an equivalent partial positive charge. This molecule may be represented as