Atomic physics is the discipline of physics concerned with phenomena related to electron configurations around atomic nuclei or inside molecules, which can be considered stable with respect to the effects of interest. Quantum physics provides an accurate model to describe these interactions. When dealing with complex particle arrangements and interactions, however, e.g. as found in stars or plasmas , statistical approaches are introduced in addition.
Bohr atomic model For the purpose of a brief introduction, it is feasible to use the simpler and planetary system-like Bohr atomic model , as illustrated on the right. It envisions a central, fixed nucleus (in the case of hydrogen a single positively charged proton p+ as references by Z = 1) with one or multiple negatively charged electrons e- circling on fixed orbitals (marked n) specific to the environment or atomic state. Higher orbitals represent higher energetic states than lower orbitals. Whenever an electron transitions from an orbital with higher energy to one with lower energy, a light particle or photon is emitted by the electron. And the energy of the photon (which in the visible spectrum determines its color) equals the energy difference between the two orbital energies involved.
Note, that the photon energy depends on its frequency
(or light color, if in the visible spectrum) or wavelength as described by the
Planck-Einstein relation: E = hν (energy = Planck constant ∙ frequency)
and ν = c / λ (frequency = constant speed of light in vaccum over wavelength).
Inversely, an electron can absorb photons of specific energies only, if the photon energy equals the amount needed to transition an electron into one of the higher energetic orbitals (but not somewhere in between orbitals).
The orbitals or energetic levels determine what energetic states electrons can assume and therefore are a characteristic of a specific atom. The energetic levels themselves are determined by the nucleus and may be disturbed by external influences like the presence of other charged or neutral particles. When the atom is disturbed, its energetic levels are also disturbed resulting in slightly different photons to be emitted or absorbed by present electrons. This causes spectral lines as shown below in Figure 6 to be widened and/or shifted and sometimes split into multiples lines.
Thus, by looking at a spectrum, a great deal of information can be gathered about the state that the electron and consequently the atom was in from which the photons originated. This is the basis for emission spectroscopy, essentially looking closely at all energy or color components of an object's electro-magnetic emmissions.