In physics, you may have learned that light has dual nature – it’s both particles as well as wave. In the similar fashion, in chemistry, every compound has both covalent as well as ionic characteristics. The compounds do have a very high percentage of ionic or covalent characteristics but 100% ionic or covalent characteristic is not present in any compound. It is an ideal situation. In reality, even in H-H bond, there is few percentage of ionic characteristic.
Bond polarity can be understood by its representation in the form of “tug-of-war” game.
In “Tug-of-war”, the two groups of people pull the rope towards themselves and away from the other. It is represented pictorially in the following figure:
Similar to the game of “tug-of-war”, in chemistry when two atoms share a pair of electrons, they try to pull it towards themselves. Now it leads to the following situations:
When both the atoms are identical:
In this case, the shared pair of electrons remains in the middle of their bond length as being identical both apply equal force over the electron.
When the two participating atoms are different:
In this case, the stronger atom (more electronegative element among the two) will attract the electrons towards itself with greater force and thus its proximity to electron pair will increase leading to an increase in negative charge over that atom. The other atom will get an equal amount of positive charge. This formation of slight charges over the atoms in covalent molecules is known as bond polarity or polar covalent bond.
Factors on which the bond polarity depends
Relative electronegativity of participating atoms:
Since the bond polarity involves pulling of electrons towards itself, hence a more electronegative element will be able to attract the electrons more towards itself. As a result, the electrons will always shift towards more electronegative element and the amount of shifting will depend upon the relative electronegativity of the participating atoms.
The spatial arrangement of various bonds in the atom:
The shared pair of electrons also experience pulling force from the other bonded and non-bonded pair of electrons. This results in differences in bond polarity between same participating atoms but present in different molecules. For e.g. Bond Polarity of O-H bond in a water molecule and acetic acid molecule differs due to the different spatial arrangement of various bonds in the molecule.
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