Atomic physics

**Atomic physics** is the field of physics studying atoms as an isolated system of electrons and an atomic nucleus. It is primarily concerned with the arrangement of electrons within the atom and the processes by which it changes. This includes ions as well as neutral atoms.

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, deeming atomic nuclei alone.

As with many scientific fields, strict delineation can be highly contrived and atomic physics is often considered in the wider context of atomic, molecular and optical physics. Physics' research groups are usually so classified.

## Isolated atoms

Atomic physics often considers atoms one by one. Atomic models will consist of a single nucleus that may be surrounded by one or more bound electrons. It is not concerned with the formation of molecules, though much of the physics is identical, nor does it examine atoms in a solid state as condensed matter. It is concerned with processes such as ionization and excitation by photons or collisions with atomic particles.

While modelling atoms in isolation may not seem realistic, if one considers atoms in a gas or plasma then the time-scales for atom-atom interactions are huge in comparison to the atomic processes that are generally considered. This means that the individual atoms can be treated as if each were in isolation, as the vast majority of the time they are.  By this consideration atomic physics provides the underlying theory in plasma physics and atmospheric physics, though both deal with very large amount of atoms.

## Electronic configuration

Electrons form notional shells around the nucleus of an atom. These are naturally in a ground state but can be excited by the energy absorption from light, or photons, magnetic flux, atomic or molecular collision. The energy required to remove an electron from its shell is called the binding energy. An amount of energy absorbed by the electron more than such amount is converted to kinetic energy according to the law of conservation of energy. An atom with an electron removed is said to have gone through ionization. Meanwhile, it is important to note that binding energy is not the same as ionization energy in that binding energy is the energy required to remove all electrons from an atom or ion while ionization energy is the energy required to remove an outermost shell electron to infinity.

In the event of electron's absorption of energy less than the binding energy, it will transition to an excited state. After a statistically sufficient amount of time, an electron in an excited state will undergo a transition to a lower state. The change in energy between the two energy levels must be accounted for. In a neutral atom, the system will emit a photon of the difference in energy.

However, if the excited atom has been previously ionized, in particular if one of its inner shell electrons has been removed, a phenomenon known as the _Auger effect_ may take place where the amount of energy is passed on to one of the bound electrons causing it to go into the continuum, allowing one to multiply ionize an atom with a single photon. There are rather strict selection rules as to the electronic configurations that can be reached by excitation by light. However, there are no such excitation rules by collision processes.

## History and developments

The majority of fields in physics can be divided between theoretical work and experimental work, and atomic physics is no exception. It is usually the case, but not always, that progress goes in alternate cycles from an experimental observation, through to a theoretical explanation followed by some predictions that may or may not be confirmed by experiment, and so on. Of course, the current state of technology at any given time can put limitations on what can be achieved experimentally and theoretically so it may take considerable time for theory to be refined.

One of the earliest steps towards atomic physics was the recognition that matter was composed of _atoms_, in the modern sense of the basic unit of a chemical element. This theory was developed by the British chemist and physicist John Dalton in the 18th century. At this stage, it wasn't clear what atoms were although they could be described and classified by their properties macroscopically in a periodic table. The true beginning of atomic physics is marked by the discovery of spectral lines and attempts to describe the phenomenon, most notably by Joseph von Fraunhofer.

The study of these lines led to the Bohr model and to the birth of quantum mechanics. In seeking to explain atomic spectra an entirely new mathematical model of matter was revealed. As far as atoms and their electron shells were concerned, not only did this yield a better overall description, i.e. the atomic orbital model, but it also provided a new theoretical basis for the chemistry's fields of quantum chemistry and atomic spectroscopy.

Since World War II, both theoretical and experimental fields have advanced at a rapid pace. This can be attributed to progress in computing technology, which has allowed larger and more sophisticated models of atomic structure and associated collision processes. Similar technological advances in accelerators, detectors, magnetic field generation and lasers have greatly assisted experimental work.

## Significant atomic physicists

- Pre quantum mechanics
- John Dalton
- J.J. Thomson
- Johannes Rydberg
- Joseph von Fraunhofer

- Post quantum mechanics
- Niels Bohr
- Max Born
- David Bates
- Enrico Fermi
- Vladimir Fock
- Alexander Dalgarno
- Clinton Joseph Davisson
- Charlotte Froese Fischer

- Nevill Mott
- Ratko Janev
- Mike Seaton
- John C. Slater
- Douglas Hartree
- Harrie S. Massey
- Ernest M. Henley
- George Paget Thomson

## Related pages

- Max Planck
- John Dalton
- Radioactivity
- Issac Newton
- Wave function
- Aerodynamics
- Albert Einstein
- Fluid dynamics
- Particle physics
- Molecular orbital
- General relativity
- Thermodynamics
- Electromagnetism
- Radioactive decay
- Erwin Schrödinger
- Classical mechanics
- Quantum mechanics

## Other websites

- Joint Quantum Institute at University of Maryland and NIST
- Atomic Physics on the Internet
- JILA (Atomic Physics)
- ORNL Physics Division