Molecular orbitals (MOs)
- Formed from linear combination of atomic orbitals (LCAO)
- Bonding orbitals result from combining in-phase orbitals
- Antibonding orbitals result from combining out-of-phase orbitals
- Number of MOs in molecule = number of AOs from atoms
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Molecular Orbital Diagram
Let's use these molecular orbitals to show how the atomic orbitals from elemental hydrogen combine to make dihydrogen (H2) in an orbital interaction diagram.
- Indicate the direction of increasing energy.
- Draw the 1s orbital of the first hydrogen on the left.
- Show the orbital as both a shape—a shaded sphere—and an energy level, a horizontal line with a single electron of "up" spin.
- Draw the 1s orbital of the second hydrogen on the right, keeping it level with the first.
- Since we have one atomic orbitals from each of the elemental hydrogens, they will combine to form two molecular orbitals in H2.
- Write that:of molecular orbitals in a molecule = # of atomic orbitals from the atoms
- Draw the bonding molecular orbital with its energy level below the 1s atomic orbitals.
- Add the shape, which is a shaded ellipsoid.
- Draw the antibonding molecular orbital at an energy level above the atomic orbitals.
- Add its shape, which is a shaded half-sphere next to an unshaded half-sphere.
- To indicate the contributions of the atomic orbitals to each molecular orbital:
- Add a dotted line from each of the 1s orbitals down to the bonding orbital.
- Add a dotted line from each of the 1s orbitals up to the antibonding orbital.
Now that the diagram has been constructed, let's put the electrons from the hydrogen atoms in the available molecular orbital.
- Draw both electrons in the bonding molecular orbital with paired spins.
- We can see that this configuration corresponds to the lowest-energy state because it matches with the Aufbau principle and the Pauli exclusion principle from earlier.