Molecular Structure and Bonding
Molecular structure and bonding describe how atoms combine into molecules through shared electrons, and how the separation of nuclear and electronic motion makes molecular quantum mechanics tractable.
Definition
Molecular structure and bonding is the study of how electrons bind nuclei into stable molecules and of the resulting equilibrium geometries and energy levels, based on solving the molecular Schrödinger equation within the Born–Oppenheimer separation of electronic and nuclear motion.
Scope
This area covers the quantum-mechanical basis of molecules: the Born–Oppenheimer approximation that separates fast electronic motion from slow nuclear motion and defines potential-energy surfaces; the theories of chemical bonding, including molecular-orbital and valence-bond pictures; and the rotational and vibrational motion of the nuclei on the resulting surfaces. It explains molecular geometry, bond formation, and the energy-level structure underlying molecular spectroscopy.
Sub-topics
Core questions
- How does the large mass difference between nuclei and electrons let us separate their motions?
- What holds atoms together in a molecule, and how is a chemical bond described quantum-mechanically?
- How do molecular orbitals form from atomic orbitals?
- How do the nuclei move—rotating and vibrating—on the electronic potential-energy surface?
Key concepts
- Born–Oppenheimer separation
- Potential-energy surface
- Molecular orbitals (LCAO)
- Bonding and antibonding orbitals
- Bond order and bond length
- Vibrational and rotational levels
Key theories
- Born–Oppenheimer approximation
- Because nuclei are far heavier than electrons, the electronic Schrödinger equation is solved for fixed nuclei to give a potential-energy surface, on which the nuclei then move; this separation underlies essentially all of molecular structure theory.
- Molecular-orbital theory
- Molecular orbitals built as linear combinations of atomic orbitals delocalize electrons over the whole molecule, with bonding and antibonding combinations explaining bond order, stability, and magnetic properties.
- Rotational–vibrational structure
- On a given electronic surface the nuclei vibrate near equilibrium and rotate as a whole, giving a ladder of vibrational levels each carrying a manifold of rotational levels that organizes molecular spectra.
Clinical relevance
Understanding molecular structure and bonding underlies all of chemistry and materials science—predicting reactivity, geometry, and spectra—and the potential-energy surfaces it defines are the starting point for computational chemistry, drug design, and the interpretation of every form of molecular spectroscopy.
History
Quantum mechanics was applied to molecules almost immediately after its formulation: Heitler and London treated the hydrogen molecule in 1927, the same year Born and Oppenheimer justified separating nuclear and electronic motion. Hund and Mulliken then developed molecular-orbital theory, and Pauling elaborated the complementary valence-bond picture of the chemical bond.
Key figures
- Max Born
- Robert Oppenheimer
- Friedrich Hund
- Robert Mulliken
Related topics
Seminal works
- born1927
- atkins2011
- bransden2003
Frequently asked questions
- Why is the Born–Oppenheimer approximation so good?
- Nuclei are thousands of times heavier than electrons, so electrons adjust almost instantly to any nuclear configuration. Treating the nuclei as fixed when solving for the electrons introduces only a small error, except near points where electronic states become degenerate.
- What is the difference between molecular-orbital and valence-bond theory?
- Molecular-orbital theory builds orbitals delocalized over the whole molecule, while valence-bond theory describes bonds as localized electron pairs shared between specific atoms. Both are approximations to the same exact wavefunction and can be reconciled.