(a) What is the difference between hybrid orbitals and molecular orbitals? (b) How many electrons can be placed into each MO of a molecule? (c) Can antibonding molecular orbitals have electrons in them?

Hybrid orbitals are formed when atomic orbitals combine or mix within the same atom to create a set of orbitals with equal energy that are more suitable for bonding. This process, known as hybridization, results in the formation of orbitals that are characterized by specific geometric shapes. For instance, the sp hybridization involves the mixture of one s orbital and one p orbital, resulting in two equivalent sp hybrid orbitals with a linear shape. On the other hand, molecular orbitals are formed when atomic orbitals of two or more atoms in a molecule combine to create a new set of orbitals that are spread over the entire molecule. Molecular orbitals account for the electron density distribution within a molecule and are characterized by either bonding or antibonding properties. Bonding molecular orbitals have lower energy and greater electron density between the nuclei, promoting bond formation. Antibonding molecular orbitals have higher energy and decreased electron density between the nuclei, thus less favorable for bond formation.

Each molecular orbital can hold a maximum of two electrons, following the Pauli Exclusion Principle. The principle states that no two electrons in a system can have the same set of quantum numbers. Since electrons in a molecular orbital can have opposite spins, the maximum allowed number of electrons in each molecular orbital is two, with one having spin up (+1/2) and the other having spin down (-1/2).

Yes, antibonding molecular orbitals can have electrons in them. Although antibonding orbitals have higher energy and are less favorable for bond formation, they can still hold electrons if there are no other available lower-energy orbitals. When occupied, the electrons in the antibonding molecular orbitals work to "cancel out" the bonding effect of the electrons in the bonding molecular orbitals, which may result in a weakened bond or even no bond in some cases. This phenomenon can be observed in molecular orbital diagrams, where the difference between the number of electrons in bonding and antibonding orbitals correlates with the bond order of the molecule.