Question

Ethylene is a planar molecule whereas acetylene is a linear molecule. Why?

Answer 1

The molecular geometry about a carbon atom in an organic molecule depends on the number of atoms bonded to the carbon. The geometry is tetrahedral for four, trigonal planar for three, and linear for two atoms bonded to carbon. In ethylene there are three atoms bonded to the carbon atom; including 2 hydrogen atom and 1 carbon atom) so the geometry is planar.  

In acetylene there are 2 atoms bonded to the carbon atom (1 hydrogen atom and 1 carbon atom) so the geometry is linear.

Answer 2

Answer: In case of ethylene C2 H4 show Sp2 hybridization where the four hydrogen atoms are placed in four corners of a plane sharing 120°Whereas acetylene shows sp hybridization and shares an angle of 180° and thus it is linear