Depending upon the nature of the atoms which combine with each other by mutual sharing of electrons, the following two types of covalent bonds are formed.
1. Non-polar covalent bond:
This type of bond is formed between the two atoms of the same element. Atoms of the same element attract electrons equally. So, when a covalent bond is formed between two atoms of the same element; the electrons are shared equally between the two atoms. In other words, the shared electron-pair will lie exactly midway between the two atoms. This type of covalent bond is described as a non-polar covalent bond. Non-polar covalent bonds are formed in the molecules such as H2, O2 and Cl2 etc.
2. Polar covalent bond:
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This type of bond is formed between two atoms of different elements; the shared pair of electrons does not lie exactly midway between the two atoms. In facts, and it lies more towards the atom which is more electronegative. The atom with higher affinity for electrons develops a slight negative charge and atom with lesser affinity for electrons a slight positive charge.
In the hydrogen chloride (HCl) molecule, hydrogen and chlorine atoms are bonded through electrons sharing.
The shared pair of electrons lies more towards Cl atom (because Cl is more electronegative). Therefore, Cl atom acquires a slight negative charge (-δ) and H atom, a slight positive charge (+δ).
The O-H bond in water (H2O) is also a polar bond – the hydrogen atom has a slight positive charge, and oxygen atom has been the slight negative charge, i.e., H+δ– O-δ. The compounds having polar covalent bonds undergo ionization when dissolved in any polar solvent, such as water. Such polar compounds in solutions produce free ions. For example, when hydrogen chloride (HCl) is dissolved in water, it ionizes to produce H3O+ (hydronium ion) and Cl– (chloride ion).
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H2O + H+δ → Cl-δ → H2O….. H+δ…… Cl-δ→ H3O+ + Cl–