Key Trend
As bond order increases: bond length decreases and bond strength (enthalpy) increases. This is because more shared electron pairs pull the nuclei closer together.
C–C: 154 pm, 346 kJ mol⁻¹ | C=C: 134 pm, 614 kJ mol⁻¹ | C≡C: 120 pm, 839 kJ mol⁻¹
Think About It
A C=C double bond (614 kJ mol⁻¹) is not twice as strong as a C–C single bond (346 kJ mol⁻¹). Why?
The first bond (σ bond) is a strong head-on overlap. The second bond (π bond) involves sideways overlap of p-orbitals, which is less effective. So the π bond adds only ~268 kJ mol⁻¹, not another 346.