Answer
The reason why all six atoms in C2H4 must lie in the same plane is due to the formation of the pi-bond in the molecule.
Work Step by Step
The ethylene (C2H4) molecule consists of two carbon atoms connected by a double bond, which includes one sigma bond and one pi bond. The pi bond is a type of bond that forms in a single plane, meaning it cannot be bent or distorted. Therefore, all the remaining atoms bonded to the carbon atoms in the ethylene molecule must also lie in the same plane.
