Problem 1 - Chemistry An Atoms First Approach

Question

What are molecular orbitals? How do they compare with atomic orbitals? Can you tell by the shape of the bonding and antibonding orbitals which is lower in energy? Explain.

Step-by-Step Solution

Answer

Molecular orbitals are regions in a molecule where atomic orbitals overlap, and electrons are likely to be found. They differ from atomic orbitals, which exist around single atoms. Bonding orbitals lead to increased molecular stability, while antibonding orbitals weaken chemical bonds and decrease stability. Bonding orbitals are generally lower in energy, have a greater electron density between nuclei, and result from the constructive combination of atomic orbitals. In contrast, antibonding orbitals are higher in energy, have a nodal plane between nuclei, and result from their destructive combination.

Step 1: Definition of Molecular Orbitals

Molecular orbitals are the regions in a molecule where atomic orbitals of participating atoms have combined and overlap, in which electrons are most likely to be found. These orbitals result from the linear combination of atomic orbitals, and they form the basis of molecular orbital theory, which helps in understanding the electronic structure of molecules.

Step 2: Comparison with Atomic Orbitals

Atomic orbitals are the regions around a single atom where electrons are likely to be found. In contrast, molecular orbitals exist in molecules and are formed by the combination of atomic orbitals from different atoms. A significant difference between atomic and molecular orbitals is the central force attracting electrons – in atomic orbitals it is needed to consider only one nucleus, whereas in molecular orbitals there are at least two.

Step 3: Bonding and Antibonding Orbitals

Bonding orbitals are molecular orbitals that, when the electrons occupy them, result in an increased stability of the molecule. They are normally formed when atomic orbitals with similar energies and shapes combine constructively. Antibonding orbitals, on the other hand, are formed when atomic orbitals with similar energies and shapes combine destructively. Electrons occupying these antibonding orbitals weaken the chemical bonds and decrease molecular stability.

Step 4: Energy and Shape of Bonding and Antibonding Orbitals

Bonding molecular orbitals are generally lower in energy than the parent atomic orbitals, making the molecule more stable when electrons occupy these orbitals. In contrast, antibonding molecular orbitals are higher in energy than the atomic orbitals. Electrons in antibonding orbitals reduce stability and weaken the bond. The shape of a molecular orbital depends on the relative orientation and shapes of the atomic orbitals, which combine to form it. The shapes of bonding and antibonding orbitals can be determined by their symmetry and nodal properties. In general, bonding orbitals exhibit a greater electron density between the nuclei (attractive force) whereas antibonding orbitals have a nodal plane (repulsive force) between the nuclei, resulting in decreased electron density in this region. In summary, molecular orbitals are formed by the combination of atomic orbitals, leading to bonding and antibonding orbitals. The shapes of these orbitals and their energetics can be linked to the contributing atomic orbitals. Bonding orbitals are lower in energy and have a greater electron density between nuclei, while antibonding orbitals are higher in energy and have a nodal plane between nuclei.