There are two types of bonds that can be formed
- Either by sharing of electrons , that is covalent bond.
Or 2. By transfer of electrons , that is ionic bond.
Now the question is that , why to go for covalent bond?
An important aspect of ionic bond is that the difference of electronegativity between the two elements must be greater than 2 for it to be ionic bond.
For example in case of CaO the electronegativity difference is (Calcium 1) – (Oxygen 3.44 ) = 2.44 . so it is ionic. Similarly in case of HI (hydrogen iodide) the difference is 0.46 , thus it is covalent in nature.
- The elements that can not gain or loose electrons to complete the valency basically go for sharing of electrons.
As in case of carbon , its valency is 4 .Neither it can gain 4 more electrons to complete its octet nor loose. So it has to make only the covalent bonds.
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