Final answer:
Propyne, or HCCCH₃, has a linear part with a C∞ rotational symmetry and a horizontal mirror plane, placing it in the C∞ᵥ point group.
Step-by-step explanation:
To determine the point group for HCCCH₃, also known as propyne, we need to look at the symmetries of the molecule. Propyne has a linear segment (HCC) and a methyl group (CH₃) attached to it. The molecule has a linear part, so there are infinite rotation axes along the carbon-carbon triple bond, giving us a C∞ element of symmetry. It also has a horizontal mirror plane (σh) due to the reflection symmetry with respect to the plane of the molecule. Therefore, based on its elements of symmetry, propyne belongs to the C∞ᵥ point group, which includes molecules with a C∞ axis and a perpendicular mirror plane.