Here's why:

* Structure: Acetylene has a linear structure. The two carbon atoms are triple-bonded to each other, and each carbon atom is also single-bonded to a hydrogen atom.

* Hybridization: The carbon atoms in acetylene are sp hybridized. This means that each carbon atom has two sp hybrid orbitals and two unhybridized p orbitals.

* Bond Formation:

* The two sp hybrid orbitals on each carbon atom form the σ bonds to the other carbon and the hydrogen atom.

* The two unhybridized p orbitals on each carbon atom overlap side-by-side to form two π bonds, resulting in the triple bond between the carbon atoms.

* Linearity: The sp hybrid orbitals lie at 180° to each other, and the p orbitals are also aligned in a linear fashion. This leads to a linear molecular geometry with a bond angle of 180°.