Question

Define the following terms: bonding molecular orbital, antibonding molecular orbital, pi molecular orbital, sigma molecular orbital.

Short answer

Bonding molecular orbitals stabilize molecules; antibonding ones destabilize them. Sigma orbitals arise from axial overlap, while pi orbitals form from lateral overlap.

Step-by-step solution

Understanding Molecular Orbitals

Molecular orbitals are formed by the overlap of atomic orbitals when atoms combine to form molecules. These orbitals belong to the entire molecule rather than a single atom and explain the binding nature in molecules.

Defining Bonding Molecular Orbitals

A bonding molecular orbital is formed when atomic orbitals constructively interfere (i.e., their wave functions add up), resulting in increased electron density between the atomic nuclei. This increased electron density holds the nuclei together, creating a stable bond.

Defining Antibonding Molecular Orbitals

An antibonding molecular orbital is formed when atomic orbitals destructively interfere (i.e., their wave functions subtract), leading to decreased electron density between the nuclei. This decreased electron density can destabilize the molecule if electrons occupy these orbitals.

Defining Sigma (σ) Molecular Orbitals

Sigma molecular orbitals are bonding or antibonding orbitals formed by head-to-head overlap of atomic orbitals, such as s-s, s-p, or p-p orbitals. This type of overlap is along the axis connecting the two nuclei, leading to cylindrical symmetry around the bond axis.

Defining Pi (π) Molecular Orbitals

Pi molecular orbitals result from the side-to-side overlap of p orbitals. These orbitals have a node along the bond axis and form above and below this axis. They typically have a weaker overlap compared to sigma orbitals, but they are crucial in double and triple bonds, where pi bonds accompany a sigma bond.

Key concepts

Bonding Molecular Orbital

Bonding molecular orbitals are crucial components in the formation of chemical bonds in molecules. These orbitals occur as a result of constructive interference when atomic orbitals align in a manner that their wave functions combine positively. This type of alignment enhances the electron density between atomic nuclei, which ultimately results in a net attraction between the atoms involved. This attraction leads to the formation of a stable chemical bond, helping the atoms to stay together and form a molecule.

In essence:

• Constructive interference increases electron density between nuclei.
• Provides stability by lowering energy of the system.
• Results in stronger chemical bonds.

Bonding molecular orbitals play a significant role in dictating the overall shape and stability of molecules, as they essentially describe how atoms within a molecule are chemically bonded together.

Antibonding Molecular Orbital

Antibonding molecular orbitals, on the other hand, result from destructive interference where the wave functions of atomic orbitals overlap in a manner that reduces electron density between the nuclei. This decrease in electron density creates a repulsive interaction instead of an attractive one, which can destabilize the molecule. In simple terms, if electrons occupy these orbitals, they can effectively weaken the bond between atoms or even prevent the bond from forming entirely.

Key points to remember about antibonding orbitals:

• Destructive interference decreases electron density, weakening bonds.
• Occupancy of these orbitals raises the energy of the system.
• They are often denoted with a "*" symbol to distinguish from bonding orbitals.

The existence of antibonding molecular orbitals highlights the balance required in the formation of stable molecules. While bonding orbitals promote stability, the occupation of antibonding orbitals works against it.

Sigma Molecular Orbital

Sigma molecular orbitals are a type of molecular orbital formed through the head-to-head overlap of atomic orbitals along the axis connecting two nuclei. This overlap could occur between two s orbitals, an s and a p orbital, or two p orbitals. Sigma orbitals are characterized by their cylindrical symmetry around the bond axis, meaning the electron density is symmetrical along the axis between the atoms.

Important characteristics:

• Occurs through head-to-head overlap.
• Cylindrical symmetry around the bond axis.
• Forms the backbone of single bonds.

Sigma orbitals are the foundational element of most single covalent bonds in chemistry. They often serve as the "first bond" in any multiple bonding scenario, which can be complemented by other types of orbitals, such as pi orbitals, to form double or triple bonds.

Pi Molecular Orbital

Pi molecular orbitals are another essential type of molecular orbital, formed when two p orbitals engage in side-to-side overlap above and below the bond axis. These orbitals have a node along the bond axis, which is a region where there is zero probability of finding an electron. Pi orbitals generally have less overlap compared to sigma orbitals, making them weaker, but they play a vital role in the existence of multiple bonds like double and triple bonds in molecules.

Characteristics of pi orbitals include:

• Formation through side-to-side overlap of atomic p orbitals.
• Presence of a nodal plane along the bond axis.
• Essential for the formation of double and triple bonds by accompanying sigma bonds.

Although weaker than sigma bonds, pi bonds give molecules like alkenes and alkynes their unique chemical properties, adjusting reactivity and interaction capabilities.