How do the shapes of ethylene and acetylene molecules differ?

The molecular shapes of ethylene and acetylene differ due to the arrangement of their bonding electron pairs around the carbon atoms.1

• Ethylene ($C_2{H}_4$): Each carbon atom is bonded to two hydrogen atoms and the other carbon atom. These three bonding regions adopt a planar triangular arrangement, resulting in a flat, planar molecular structure with bond angles close to 120°. The presence of a double bond prevents free rotation, further stabilizing this rigid, planar geometry.12
• Acetylene ($C_2{H}_2$): Each carbon atom is bonded to one hydrogen atom and the other carbon atom via a triple bond. Because there are only two bonding regions around each carbon atom, the molecule adopts a linear shape.13