Why the angle between c-c in ethylene is less than 120 as it is having sp2 hybridisation

The angle between the carbon-carbon bond in ethylene (C2H4) is actually approximately 120 degrees, which is consistent with the geometry of sp2 hybridization. In sp2 hybridization, the s orbital and two p orbitals of the carbon atom hybridize to form three sp2 hybrid orbitals. These orbitals are arranged in a trigonal planar geometry, which has bond angles of approximately 120 degrees.

However, slight variations in the bond angle can occur due to the presence of lone pairs of electrons or differences in electronegativity between the atoms involved in the bond. But in the case of ethylene, there are no lone pairs on the carbon atoms and the atoms involved in the bonds (carbon and hydrogen) have similar electronegativities. Therefore, the bond angle should be close to 120 degrees.

If you have a source that states the bond angle in ethylene is significantly less than 120 degrees, it may be incorrect or there may be additional factors at play that are not included in a basic analysis of the molecule's geometry.