Chemistry Bonding and antibonding molecular orbitals

[henry3369]

Not a homework question, just curious. My book says that bonding orbitals from from constructive interference while antibonding orbitals from destructive interference. Since constructive interference increases amplitude, what increases in amplitude from the combination of bonding orbitals? It can't be energy because bonding orbitals are lower in energy.

 

[Suraj M]

Stability? What actually happens is that the bonding orbitals formed by constructive interference is formed between the nuclei of the reacting atoms/molecules.. so they hold the molecule together increasing stability! whereas antibonding is away from both nuclei, hence not helping in holding the molecule together!

 

[DrClaude]

Since constructive interference increases amplitude, what increases in amplitude from the combination of bonding orbitals?

The wave function.

The statement stems from building molecular orbitals (MO) by the Linear Combination of Atomic Orbitals (LCAO) method, where you would take an orbital ($\phi$) on each atom and make a linear combination to build the MO ($\psi$). For two atoms A and B, you get
$$
\begin{align}
\psi_+ &= \frac{1}{\sqrt{2}} \left( \phi_A +\phi_B \right) \\
\psi_- &= \frac{1}{\sqrt{2}} \left( \phi_A - \phi_B \right)
\end{align}
$$
In the case of $\psi_+$ you get "constructive interference," as the two atomic orbitals $\phi$ add up, and "destructive interference" for $\psi_-$. As the atomic orbitals are centered on different atoms, this interference takes place in between the atoms, such that the electron density is increased ($\psi_+$) or decreased and has a node ($\psi_-$) between the nuclei, leading to a reduction or an increase in the nuclear Coulomb repulsion, respectively.