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🥼organic chemistry review

Sigma Star

Written by the Fiveable Content Team • Last updated September 2025
Written by the Fiveable Content Team • Last updated September 2025

Definition

Sigma star (σ*) is a type of molecular orbital that is formed by the constructive interference of atomic orbitals in a molecule. It represents the antibonding interaction between atoms, which weakens the bond between them and increases the energy of the overall molecular system.

5 Must Know Facts For Your Next Test

  1. Sigma star (σ*) orbitals are antibonding in nature, meaning they weaken the bond between the atoms involved.
  2. Sigma star orbitals are formed by the constructive interference of atomic orbitals that are out of phase, leading to a decrease in electron density between the bonded atoms.
  3. Occupying sigma star orbitals with electrons increases the overall energy of the molecular system, making the molecule less stable.
  4. Sigma star orbitals have a higher energy level compared to bonding sigma (σ) orbitals, as they represent a higher energy state of the molecule.
  5. The presence of sigma star orbitals in a molecule can influence its reactivity and stability, as they can participate in various chemical reactions and transformations.

Review Questions

  • Explain the role of sigma star (σ*) orbitals in the context of molecular orbital theory.
    • In the molecular orbital theory, sigma star (σ*) orbitals represent the antibonding interactions between atoms in a molecule. These orbitals are formed by the constructive interference of atomic orbitals that are out of phase, leading to a decrease in electron density between the bonded atoms. Occupying sigma star orbitals with electrons increases the overall energy of the molecular system, making the molecule less stable. The presence of sigma star orbitals can influence the reactivity and stability of a molecule, as they can participate in various chemical reactions and transformations.
  • Describe how sigma star (σ*) orbitals differ from bonding sigma (σ) orbitals in terms of their properties and effects on chemical bonds.
    • Sigma star (σ*) orbitals and bonding sigma (σ) orbitals have contrasting properties and effects on chemical bonds. Bonding sigma orbitals result from the constructive interference of atomic orbitals, leading to an increase in electron density between the bonded atoms and strengthening the chemical bond. In contrast, sigma star orbitals arise from the destructive interference of atomic orbitals, causing a depletion of electron density between the bonded atoms and weakening the chemical bond. Occupying sigma star orbitals with electrons increases the overall energy of the molecular system, making the molecule less stable, while filling bonding sigma orbitals decreases the energy of the molecule and stabilizes the chemical bond.
  • Analyze the significance of sigma star (σ*) orbitals in the context of chemical reactivity and stability, and explain how their presence can influence the behavior of a molecule.
    • Sigma star (σ*) orbitals play a crucial role in determining the reactivity and stability of a molecule. As antibonding orbitals, the presence of electrons in sigma star orbitals weakens the chemical bonds, making the molecule more reactive and less stable. This increased reactivity can lead to various chemical transformations, such as bond cleavage, rearrangements, and participation in chemical reactions. The energy level of sigma star orbitals is higher compared to bonding sigma (σ) orbitals, and occupying these orbitals with electrons raises the overall energy of the molecular system, destabilizing the molecule. Understanding the influence of sigma star orbitals on molecular properties is essential in predicting and explaining the behavior of molecules in chemical processes, including their reactivity, stability, and potential for undergoing transformations.