Question

When applying molecular orbital theory to heteronuclear diatomic molecules, the atomic orbitals used may be of different energies. If two atomic orbitals of different energies make two molecular orbitals, how are the energies of the molecular orbitals related to the energies of the atomic orbitals? How is the shape of the resultant molecular orbitals related to the shapes of the atomic orbitals?

Short answer

The energy of the bonding molecular orbital is between the energies of the two atomic orbitals, while the energy of the antibonding molecular orbital is higher than both. The shape of the molecular orbitals will have increased contribution from the atomic orbital of the atom with higher electronegativity.

Step-by-step solution

Understanding Molecular Orbital Theory for Heteronuclear Diatomic Molecules

Molecular Orbital (MO) Theory states that atomic orbitals combine to form molecular orbitals, which are delocalized over the entire molecule. For heteronuclear diatomic molecules, the atomic orbitals that combine can be of different energies due to the different atoms involved.

Combination of Atomic Orbitals of Different Energies

When atomic orbitals of different energies combine, they create two molecular orbitals: a bonding molecular orbital (lower energy) and an antibonding molecular orbital (higher energy). The energy of the bonding molecular orbital will be lower than the higher-energy atomic orbital but higher than the lower-energy atomic orbital, while the energy of the antibonding molecular orbital will be higher than both atomic orbitals.

Results of Molecular Orbital Formation on Orbital Shapes

The shape of the resultant molecular orbitals will be influenced by the shapes of the original atomic orbitals. However, the contribution of each atomic orbital to the molecular orbital will depend on their relative energies and the difference in energy levels. The molecular orbital will more closely resemble the atomic orbital of the atom with higher electronegativity (lower energy atomic orbital).

Key concepts

Heteronuclear Diatomic Molecules

Imagine two dancers with different dance styles coming together to perform a routine. In chemistry, this is akin to the forming of heteronuclear diatomic molecules. These are molecules composed of two different kinds of atoms, like oxygen and hydrogen in water (H2O). When these distinct atoms join, the atomic orbitals (their 'dance moves') blend to form molecular orbitals.

In the world of molecules, electrons exist in regions called orbitals, which are distinctive in shape and energy. Every atom brings its unique set of orbitals to the dance floor, and when different atoms (heteronuclear) bond, their orbitals mix, creating a molecular choreography that adheres to specific physical laws, one of which is the conservation of orbitals. This means if two atomic orbitals come together, two molecular orbitals are formed. A fascinating aspect is how the new energy levels of these molecular orbitals relate to the original atomic orbital energies, which we will explore in the next segments.

Bonding Molecular Orbital

The bonding molecular orbital is like a harmonious duet between two atoms, offering stability and lower energy—music to the electrons' ears! This bonding molecular orbital forms when atomic orbitals overlap in a constructive manner. Think of it as the overlapping sound waves from two tuning forks that create a resonant, pleasant tone.

Electrons here are in a more stable condition compared to when they were in the separate atomic orbitals, and they serve like the 'glue' holding atoms together. The fascinating part of this is that the energy of the bonding orbital lies lower than the higher-energy atomic orbital but remains higher than the lowest-energy atomic orbital. This position affords a 'sweet spot' that gives bonding orbitals their stability-enhancing properties. By understanding this, students can envision how the formation of a bonding molecular orbital leads to a stable molecule.

Antibonding Molecular Orbital

Complementing the theme of stability is the antibonding molecular orbital, which is akin to dissonance in our atomic symphony. These orbitals form when atomic orbitals overlap in a manner that their electron clouds experience destructive interference, similar to noise-cancelling headphones cancelling out sound.

Electrons residing in antibonding orbitals contribute to instability and higher energy, which is not ideal for the atoms involved. The key is that the energy of an antibonding molecular orbital is higher than that of both contributing atomic orbitals. An antibonding molecular orbital effectively raises the energy of the system, leading to a less favorable, often weaker and more reactive, bonding situation. Recognizing antibonding orbitals is crucial for students to comprehend why certain molecular formations are less stable and more reactive.

Orbital Shapes

Orbitals are the 'dance floors' on which electrons move, and their shapes are vital to understanding molecular interactions. Atomic orbitals can be shaped like spheres, dumbbells, or even more complex forms, and when they combine to form molecular orbitals, the resulting shapes reflect the original atomic shapes.

The contributions of each atomic orbital are not equal; the final dance move (molecular orbital shape) tends to favor the atom with the lower-energy atomic orbital—usually the more electronegative one. Just like a dominant dance partner can influence the performance's style, the electronegative atom's orbital makes a larger contribution to the shape of the molecular orbital. Understanding the way these shapes blend and influence bonding can help elucidate a molecule's properties, such as its bonding strength and angle, polarity, and how it might interact with other molecules. This insight is pivotal for students delving into molecular geometry and reactivity.