Atomic physics studies atoms as an isolated system of electrons and an atomic nucleus and is primarily concerned with the arrangement of electrons around the nucleus and the processes by which these arrangements change. This comprises ions, neutral atoms and, unless otherwise stated, it can be assumed that the term atom includes ions.
Thumbnail: 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. Image used with permission (CC-SA-BY-3.0; JabberWok).
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