Introduction into Molecular Physics
From Atoms to Molecules
Having understood how electrons behave in atoms, we now extend our view to molecules — systems in which two or more atoms share or exchange electrons. The transition from isolated atoms to bound molecules introduces new layers of complexity: the interaction between nuclei, the redistribution of electron density, and the resulting collective quantum motion that defines chemical bonds.
At the molecular level, quantum mechanics becomes chemistry. The same principles that determine atomic orbitals now explain bond formation, bond strength, and molecular geometry. Moreover, because molecules possess internal degrees of freedom — rotation, vibration, and electronic excitation — their spectra reveal rich fingerprints that connect theory to observation.
Scope of this Section
This module provides the foundation for understanding molecular structure and spectroscopy. Building on atomic physics, we will explore how electrons and nuclei together determine molecular properties.
By the end of this module, you should be able to:
- Explain how chemical bonds arise from the interplay of attractive and repulsive interactions between atoms.
- Describe the Born–Oppenheimer approximation and how it separates electronic and nuclear motion.
- Understand the concept of potential energy curves and how they determine molecular stability.
- Construct simple molecular orbitals using the linear combination of atomic orbitals (LCAO) approach.
- Analyze the rotational and vibrational motion of diatomic molecules.
- Distinguish between infrared and Raman spectroscopy, and understand what each reveals about molecular structure.
Structure of this Module
- Foundations of Molecular Bonding – Why molecules form: qualitative picture of bonding, potential energy curves, and the idea of bonding vs. antibonding orbitals.
- The H\(_2^+\) Molecular Ion – The simplest possible molecule: exact solution and insights into electronic bonding.
- Molecular Orbitals and the LCAO Method – Building approximate molecular orbitals from atomic ones.
- Beyond H\(_2^+\) and LCAO – Refinements and limitations of simple bonding models.
- Rotation and Vibration – Nuclear motion on potential energy surfaces: rotational and vibrational quantization.
- Molecular Transitions and Spectroscopy – How light interacts with molecules: selection rules, infrared absorption, and Raman scattering.
Further Reading and Connections
To deepen your understanding, you may consult:
- B.H. Bransden and C.J. Joachain, Physics of Atoms and Molecules — Chapters on molecular bonding and spectroscopy.
- J. Levine, Quantum Chemistry — Conceptual and mathematical treatment of bonding and molecular orbitals.
- P. Atkins & R. Friedman, Molecular Quantum Mechanics — Detailed discussion of potential energy surfaces and the Born–Oppenheimer approximation.
- C.N. Banwell & E.M. McCash, Fundamentals of Molecular Spectroscopy — A concise and clear guide to infrared and Raman spectra.
Key idea:
Molecules are quantum systems of coupled electrons and nuclei. Their structure and spectra emerge from the same principles as atoms — but now enriched by the physics of bonding, motion, and interaction.