Describe the structure of the carbon-carbon double bond in alkenes.

(N/A) The carbon-carbon double bond in alkenes consists of one $\sigma$-bond and one $\pi$-bond.
$\sigma$-bond is formed by the head-on overlapping of $sp^{2}$ hybridized orbitals.
$\pi$-bond is formed by the lateral (sideways) overlapping of unhybridized $2p$ orbitals.
$1$ Double bond $= (1 \sigma + 1 \pi \text{ bond})$.
$\sigma$-bond enthalpy $\approx 397 \ kJ \ mol^{-1}$.
$\pi$-bond enthalpy $\approx 284 \ kJ \ mol^{-1}$.
$\sigma$-bond between $2$ carbon atoms: $(i)$ The double bond contains $1 \sigma$-bond between carbon atoms which are $sp^{2}$ hybridized. $(ii)$ The enthalpy of the $\sigma$-bond is $397 \ kJ \ mol^{-1}$.
$\pi$-bond between $2$ carbon atoms: $(i)$ Two carbon atoms have a $\pi$-bond between them formed by the overlapping of $2p$ orbitals. $(ii)$ The weaker $\pi$-bond has an enthalpy of $284 \ kJ \ mol^{-1}$. Since the $\pi$-bond is weaker, alkenes are more reactive than alkanes.
In alkenes, the carbon-carbon bond length is $134 \ pm$, which is shorter than the $C-C$ single bond length of $154 \ pm$.