Modern Quantum Theory is the theoretical foundation of physics which helps explain the chemical behavior of matter at both the atomic and subatomic levels. Quantum theory focuses on the application of quantum mechanics within chemical systems, relying on both experimental and theoretical methods.
Today, quantum chemists use many different methods to investigate the quantum nature of chemical phenomena. One such method is spectroscopy, which can calculate and determine the quantization of energy within an atom. Since molecules have discrete energy levels, quantum chemists can use this data to determine the state of atoms, whether they are in the ground state, excited state, or even in transition states when they undergo different chemical reactions. Another popular method is Nuclear Magnetic Resonance Spectroscopy, which exploits the magnetic properties of atomic nuclei. NMR Spectroscopy can determine the structure, chemical environment, and the reaction state of a particular molecule, just by analyzing the properties of its nuclei.
The modern focus of quantum theory in chemistry is the electronic structure of a molecule or crystal, as a molecule’s electronic structure will ultimately determine how it behaves in different conditions, and how it will react with different molecules within different chemical systems.
Thus, studying modern quantum theory in the context of chemical systems is essential in constructing a conceptual framework needed to understand the atomic and subatomic behavior of atoms, molecules, and condensed matter.