(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 by the mixing of atomic orbitals within the same atom, leading to a redistribution of the electron density in a specific shape and orientation. This process is known as hybridization and helps explain the geometry and bond angles of molecules. Common types of hybrid orbitals are sp, sp2, and sp3. Molecular orbitals, on the other hand, are formed by the combination of atomic orbitals from different atoms when they come close together, and electrons are shared between the two nuclei. The resulting molecular orbitals are associated with the entire molecule. Molecular orbitals are formed through the Linear Combination of Atomic Orbitals (LCAO) method and are usually classified as bonding orbitals (lower in energy) and antibonding orbitals (higher in energy).

Each molecular orbital (MO) can accommodate up to two electrons, with opposite spins (one with spin up and the other with spin down). Molecular orbitals are filled according to the Aufbau Principle, which states that electrons will fill orbitals of lower energy first and follow Hund's Rule (maximum multiplicity) and the Pauli Exclusion Principle (no two electrons may have the same set of quantum numbers).

Yes, antibonding molecular orbitals can have electrons in them. However, the presence of electrons in antibonding orbitals weakens the bond strength between the atoms involved, as these electrons are located in an area of higher energy, causing an increased repulsion between the electron clouds. Electrons fill the bonding orbitals before the antibonding orbitals if the atoms are forming a stable molecule. If the molecule has a greater number of electrons in antibonding orbitals than bonding orbitals, the overall bond order will be less than or equal to zero, and the atoms may not hold together, resulting in an unstable species.