Question

why sigma bond is strong and pi bond is weak ?​

Answer 1

Sigma bonds are significantly stronger than pi bonds. This is because sigma bonds allow for electron density to be concentrated to a much larger degree between the two nuclei. You can also conceptualize that pi bonds are weaker simply because we know those electrons are in a higher-energy state.

Answer 2

• A pi bond is a weaker chemical covalent bond than a sigma bond (since π bonds have a smaller overlap between the orbitals), but when it is put with a sigma bond it creates a much stronger hold between the atoms, thus double and triple bonds are stronger then single bonds.