Non-polar covalent bond
When a covalent bond is formed between two atoms of the same element, the shared electron pair will lie exactly midway between the two atoms i.e. the electrons are equally shared by the atoms. The resulting molecule will be electrically symmetrical i.e., centre of the negative charge coincides with the centre of the positive charge. This type of covalent bond is described as a non-polar covalent bond. The bonds in the molecules H2, O2, Cl2 etc., are non-polar covalent bonds.
Polar covalent bond
The bond between two unlike atoms, which differ in their affinities for electrons is said to be a polar covalent bond. When a covalent bond is formed between two atoms of different elements, the bonding pair of electrons will lie more towards the atom, which has more affinity for electrons. As the said electron pair do not lie exactly midway between the two atoms, the atom with higher affinity for electrons develops a slight negative charge and the atom with lesser affinity for electrons, a slight positive charge. Such molecules are called 'polar molecules'.In the hydrogen chloride (HCl) molecule, the bonding of hydrogen and chlorine atoms lies more towards Cl atom (because Cl is more electronegative) in the shared pair of electrons. Therefore, Cl atom acquires a slight negative charge, and H atom a slight positive charge. This causes the covalent bond between H and Cl to have an appreciable ionic character.
The compounds having polar bonds are termed polar compounds. Polar substances in their pure forms, do not conduct electricity, but give conducting solutions when dissolved in polar solvents.
Cause of polarity in bonds
A pure covalent bond is formed when the shared pair of electrons is shared equally by the two atoms. Conversely when the two combining atoms share the shared pair of electrons unequally, the bond formed is a polar covalent bond. Unequal sharing of the shared pair of electrons arises due to an unequal electron-attracting tendency of the two atoms. The electron-attracting tendency of the atoms in a molecule is described in terms of electronegativity. 'Polarity in a bond arises due to the difference in the electronegativities of the combining atoms'. Thus, the atom of an element having higher electronegativity has greater electron attracting tendency.For example, two atoms of hydrogen combine to form a molecule of hydrogen in which H-H is a pure covalent bond. But, when the atoms of different elements (with different electronegativities) combine, the atom of the more electronegative element attracts the shared pair of electrons more towards it. This leads to the formation of a polar bond.
Hydrogen and chlorine react to give hydrogen chloride (HCl). HCl is a polar molecule because of the difference in the electronegativity values of hydrogen and chlorine.The greater is the difference in the electronegativity values of the combining atoms, greater is the polar character in the bond so formed.
For example, in the series H - X (X=F, Cl, Br,I), the electronegativity difference between H and X atom follows the order:H- F > H - Cl > H - Br > H - I
Therefore, the polarity in the H - X bond follows the order,
H - F > H - Cl > H - Br > H I i.e., H - F bond is the most polar andH -I bond is the least polar in this series of compounds.