Atomic physics is the field of physics that studies atoms as isolated systems that include electrons and the atomic nucleus. It focuses primarily on the arrangement of electrons around the nucleus and how it is modified. This definition encompasses both ions and electrically neutral atoms.
Since “atomic” and “nuclear” are used synonymously in everyday language, atomic physics is often confused with nuclear physics. However, physicists consider that nuclear physics focuses mainly on the atomic nucleus.
(In the Bohr model, the transition of an electron with n=3 to the shell n=2 is shown, where a photon is emitted. An electron from shell (n=2) must have been removed beforehand by ionization. )
Atomic physics always considers atoms in isolation, that is, a model is always composed of a nucleus and electrons that orbit around. It is not concerned with the formation of molecules (even if the physical theory at play is transposable), nor with the physics of the solid. It considers the processes of ionization, excitation by photons and collisions between atomic particles.
It may seem that the modeling of atoms in isolation does not reflect reality. Atoms can form a gas or a plasma, where the time scale for the interactions between atoms is enormous compared to the atomic processes of which the theory is concerned. As a result, atoms are mostly isolated in reality. It is for this reason that this theory is useful for plasma modeling and for studying the atmosphere, even if both contain huge amounts of atoms.
An electron can be excited enough to escape the attraction of the nucleus. It is said then that the rest of the atom is an ion, that is to say a quasi electrically charged atom.
Most fields in physics are either theoretical or experimental. The same is true of atomic physics. The progress of a domain is mostly based on two pillars: (1) modeling and (2) validation. If the validation cannot confirm the model, the model is to be reviewed. Similarly, if the model predicts a behavior that cannot be validated with the available tools, other tools may be developed to confirm or invalidate the model. The technology present has a major impact on the development of a model, because the more precise tools are most often derived from new technologies.
Clearly, the first essential step in the elaboration of an atomic theory is the recognition that matter is composed of atoms, in the modern sense of the basic unit of chemical elements. This theory was mainly developed by the chemist and physicist John Dalton in the eighteenth century. At that time, the notion of atom was unclear, although the properties could be described and classified in a periodic table.
The modern birth of the theory dates back to the discovery of spectral lines and attempts to describe the phenomenon, Fraunhofer standing out particularly. The study of these lines led to the construction of Bohr’s model and the birth of quantum theory. In attempting to mathematically describe the atomic spectrum, a whole new mathematical model appeared. With regard to atoms and their electron cloud, this model has not only provided a better description via the orbital atomic model, but has also given new theoretical foundations to chemistry, quantum chemistry, and spectroscopy.
Since the Second World War, both the theoretical and experimental part have progressed rapidly. This is mainly due to the emergence of computer science that has allowed to develop more sophisticated models, mathematically speaking, of the atomic structure and collision processes. Notable advances in particle accelerators, particle detectors, magnetic field generators and lasers have greatly supported research efforts.