In addition to forming single and double bonds by sharing two and four electrons, respectively, carbon can also form a triple bond by sharing six electrons. To account for the triple bond in a molecule such as acetylene, \(\mathrm{H}-\mathrm{C} \equiv \mathrm{C}-\mathrm{H}\), we need a third kind of hybrid orbital, an sp hybrid. Imagine that, instead of combining with two or three \(p\) orbitals, a carbon 2 s orbital hybridizes with only a single \(p\) orbital. Two \(s p\) hybrid orbitals result, and two \(p\) orbitals remain unchanged. The two \(s p\) orbitals are oriented \(180^{\circ}\) apart on the right-left \((x)\) axis, while the \(p\) orbitals are perpendicular on the up-down \((y)\) axis and the in-out \((z)\) axis, as shown in Figure \(\PageIndex{1}\). When two sp -hybridized carbon atoms approach each other, sp hybrid orbitals on each carbon overlap head-on to form a strong sp – sp σ bond. At the same time, the p z orbitals from each carbon form a p z – p z \(\pi\) bond by sideways overlap, and the p y orbitals overlap similarly to form a p y – p y \(\pi\) bond. The net effect is the sharing of six electrons and formation of a carbon–carbon triple bond. Each of the two remaining sp hybrid orbitals forms a σ bond with hydrogen to complete the acetylene molecule (Figure \(\PageIndex{2}\)). As suggested by sp hybridization, acetylene is a linear molecule with H–C–C bond angles of 180°. The C–H bonds have a length of 106 pm and a strength of 558 kJ/mol (133 kcal/mol). The C–C bond length in acetylene is 120 pm, and its strength is about 965 kJ/mol (231 kcal/mol), making it the shortest and strongest of any carbon–carbon bond. A comparison of sp, sp 2 , and sp 3 hybridization is given in Table \(\PageIndex{1}\). Molecule Bond Bond strength Bond strength.1 Bond length (pm) NaN NaN (kJ/mol) (kcal/mol) (kcal/mol) Methane, \(\mathrm{CH}_4\) \(\left(s p^3\right) \mathrm{C}-\mathrm{H}\) 439 105 109 Ethane, \(\mathrm{CH}_3 \mathrm{CH}_3\) \(\left(s p^3\right) C-C\left(s p^3\right)\) 377 90 153 NaN \(\left(s p^3\right) \mathrm{C}-\mathrm{H}\) 421 101 109 Ethylene, \(\mathrm{H}_2 \mathrm{C}=\mathrm{CH}_2\) \(\left(s p^2\right) \mathrm{C}=\mathrm{C}\left(s p^2\right)\) 728 174 134 NaN \(\left(s p^2\right) \mathrm{C}-\mathrm{H}\) 464 111 109 Acetylene, \(\mathrm{HC} \equiv \mathrm{CH}\) \((s p) \mathrm{C} \equiv \mathrm{C}(s p)\) 965 231 120 NaN \((s p) \mathrm{C}-\mathrm{H}\) 558 133 106 Exercise \(\PageIndex{1}\) Draw a line-bond structure for propyne, CH 3 C≡CH. Indicate the hybridization of the orbitals on each carbon, and predict a value for each bond angle. Answer The CH 3 carbon is sp 3 ; the triple-bond carbons are sp; the C≡C−C and H−C≡C bond angles are approximately 180°.