When three atoms come together to form a molecule, the molecular orbitals (MOs) are formed through the combination of atomic orbitals from each atom. This process is known as molecular orbital theory.
To simplify the explanation, let's consider the case of three atoms A, B, and C. Each atom has its own set of atomic orbitals, such as A's 1s, 2s, and 2p orbitals, B's 1s, 2s, and 2p orbitals, and C's 1s, 2s, and 2p orbitals.
The molecular orbital formation begins by combining the atomic orbitals of the three atoms in a linear combination, just like in the two-atom case. For example, the 1s orbitals of atoms A, B, and C can combine to form three molecular orbitals: a bonding molecular orbital (σ1s), a nonbonding molecular orbital (σ1s), and another nonbonding molecular orbital (σ1s). These molecular orbitals are labeled based on their symmetry and energy levels.
After the initial molecular orbitals are formed, they can overlap with the remaining atomic orbitals of the third atom. For instance, the σ1s molecular orbital formed from the combination of A's, B's, and C's 1s orbitals can overlap with the 2p orbitals of atom C. This overlap creates new molecular orbitals involving all three atoms.
The resulting molecular orbitals can exhibit different energies and symmetries, and their occupation by electrons determines the electronic structure and bonding in the molecule. The specific combination and overlap of molecular orbitals depend on the types and orientations of the atomic orbitals involved and the molecular geometry of the molecule.
It's important to note that the process of molecular orbital formation becomes more complex as the number of atoms increases, and the resulting molecular orbitals become increasingly challenging to represent and analyze. Computational methods and approximation techniques are often employed to study and predict the behavior of molecular orbitals in more complex systems.