Find the bonding and antibonding Hückel molecular orbitals for ethene.
The equations for $c_1$ and $c_2$ associated with Equation 11.7 are
$$
c_1(\alpha-E)+c_2 \beta=0 \quad \text { and } \quad c_1 \beta+c_2(\alpha-E)=0
$$
For $E=\alpha+\beta$, either equation yields $c_1=c_2$. Thus,
$$
\psi_{\mathrm{b}}=c_1\left(2 p_{z 1}+2 p_{z 2}\right)
$$

The value of $c_1$ can be found by requiring that the wave function be normalized. The normalization condition on $\psi_\pi$ gives $c_1^2(1+2 S+1)=1$. Using the Hückel assumption that $S=0$, we find that $c_1=1 / \sqrt{2}$.

Substituting $E=\alpha-\beta$ into either of the equations for $c_1$ and $c_2$ yields $c_1=-c_2$, or
$$
\psi_{\mathrm{a}}=c_1\left(2 p_{z 1}-2 p_{z 2}\right)
$$
The normalization condition gives $c^2(1-2 S+1)=1$, or $c_1=1 / \sqrt{2}$.
