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Why are atoms in an ethyne triple bond arranged in a straight line?

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representations of molecular structure — molecule Several methods of representing a molecule's structure. In Lewis structures, element symbols represent atoms, and dots represent electrons surrounding them. A pair of shared electrons (covalent bond) may also be shown as a single dash. The ball-and-stick model better illustrates the spatial arrangement of the atoms. For aromatic compounds, the Kekulé structure is common, in which each bond is represented by a dash, carbon atoms are implied where two or more lines meet, and hydrogen atoms are usually omitted. Bond-line formulas, similar to the Kekulé structure, are often used for complex nonaromatic organic compounds. Sugars are often drawn as Fischer projections, in which the carbon “backbone” is drawn as a straight vertical line, with carbon atoms implied where horizontal lines intersect the vertical one.

The atoms in an ethyne (acetylene) molecule are arranged in a straight line because of the specific hybridization of the carbon atoms involved in the triple bond.31

In ethyne ( $C_{2} H_{2}$ ), each carbon atom undergoes $s p$ hybridization, which involves the mixing of one $2 s$ orbital and one $2 p$ orbital. This process creates two equivalent $s p$ hybrid orbitals that point in opposite directions, 180° apart, to minimize electron repulsion. These hybrid orbitals form $σ$ (sigma) bonds with the hydrogen atom and the other carbon atom, establishing the linear backbone of the molecule.1

The remaining two $2 p$ orbitals on each carbon atom remain unhybridized and are oriented perpendicular to the bond axis and to each other. These orbitals overlap side-by-side with the corresponding orbitals on the adjacent carbon to form two $π$ (pi) bonds. This electronic configuration, consisting of one $σ$ bond and two $π$ bonds, dictates the linear geometry of the molecule, as observed in practice.231