Quantum chemistry is a branch of chemistry whose primary focus is the application of quantum mechanics in physical models and experiments of chemical systems. It involves heavy interplay of experimental and theoretical methods:
In these ways, quantum chemists investigate chemical phenomena.
Atomic physics is the field of physics that studies atoms as an isolated system of electrons and an atomic nucleus. It is primarily concerned with the arrangement of electrons around the nucleus and the processes by which these arrangements change. This includes ions as well as neutral atoms and, unless otherwise stated, for the purposes of this discussion it should be assumed that the term atom includes ions.
The term atomic physics is often associated with nuclear power and nuclear bombs, due to the synonymous use of atomic and nuclear in standard English. However, physicists distinguish between atomic physics — which deals with the atom as a system consisting of a nucleus and electrons — and nuclear physics, which considers atomic nuclei alone.
A chemical bond is an attraction between atoms that allows the formation of chemical substances that contain two or more atoms. The bond is caused by the electrostatic force of attraction between opposite charges, either between electrons and nuclei, or as the result of a dipole attraction. The strength of chemical bonds varies considerably; there are "strong bonds" such as covalent or ionic bonds and "weak bonds" such as dipole–dipole interactions, the London dispersion force and hydrogen bonding.
Since opposite charges attract via a simple electromagnetic force, the negatively charged electrons that are orbiting the nucleus and the positively charged protons in the nucleus attract each other. An electron positioned between two nuclei will be attracted to both of them, and the nuclei will be attracted toward electrons in this position. This attraction constitutes the chemical bond. Due to the matter wave nature of electrons and their smaller mass, they must occupy a much larger amount of volume compared with the nuclei, and this volume occupied by the electrons keeps the atomic nuclei relatively far apart, as compared with the size of the nuclei themselves. This phenomenon limits the distance between nuclei and atoms in a bond.
Theoretical chemistry seeks to provide explanations to chemical and physical observations. Should the properties derived from the quantum theory give a good account of the above mentioned phenomena, we derive consequences using the same theory. Should the derived consequences fall too far from the experimental evidence, we go to a different theory. G. Lewis proposed that chemical properties originated from the electrons of the atom's valence shell, ever since the theoretical chemistry has dealt with modelling of the outer electrons of interacting atoms or molecules in a reaction. Theoretical chemistry includes the fundamental laws of physics Coulomb's law, Kinetic energy, Potential energy, the Virial Theorem, Planck's Law, Pauli exclusion principle and many others to explain but also predict chemical observed phenomena. The term quantum chemistry which comes from Bohr's quantized model of electron in the atom, applies to both the time independent Schrödinger and the time dependent Dirac formulations.
In general one has to distinguish, theoretical approach (theory level such as Hartree-Fock (HF), Coupled cluster, Relativistic, etc.) from mathematical formalism, plane wave, spherical harmonics, Bloch wave periodic potential. Methods that solve iteratively the energies (Eigenvalues) of stationary state waves in a potential include Restricted Hartree-Fock (RHF), Multi-configurational self-consistent field (CASSCF or MCSCF) but the theory pertains to Schroedinger. Related areas in theoretical chemistry include the mathematical characterization of bulk materials (e.g. the study of electronic band structure in solid state physics) using the theory of Electronic band structure in a Periodic Crystal Lattice. Different theoretical approaches are Molecular mechanics and Topology. The study of the applicability of well established mathematical theories to chemistry is crucial to metals (i.e. topology in the study of small bodies explains the elaborate electronic structures of clusters). This later area of theoretical chemistry originates from the so-called mathematical chemistry. Time Dependent Quantum Molecular Dynamics, is a modern approach to the interaction of light with molecules that vibrate and drive reactions in a desired direction.
The Wiswesser rule gives a simple method to determine the energetic sequence of the atomic subshells (n,l). n is the principal quantum number and l is the azimuthal quantum number. The energetic sequence of the subshells characterized by the quantum numbers (n,l) is the sequence that leads to a monotonous increasing row of function values for the Wiswesser-function.
For example: if n=2 and l=1, this correspondents with an 2p-orbital.