Many physical phenomena can only be explained satisfactorily with a basic knowledge about the structure of matter. The chemical elements are substances that cannot be divided into further material components. The smallest particles of the elements that still exhibit their properties, and that cannot be further sub-divided chemically are the atoms.

Rutherford and Bohr found out that atoms consist of a positively charged nucleus and negatively charged electrons that are circling around the nucleus in defined orbits.
According to Bohr, an atom can pick up or release energy only in defined steps. In case an atom collides with another particle or a photon of appropriate energy, it can take over this energy and become excited. This excitation, be it of electrical, thermal or optical origin, expresses itself in a transition of an electron from an inner to a (more energetic) outer ‘shell’ (orbit). The absorption of photons by atoms is the basis of Atomic Absorption Spectrometry (AAS). After a few nanoseconds the electron usually returns to its original orbit and the atom into the ground state.

If the energy released in this process is emitted in the form of radiation, we are talking about (Optical) Atomic Emission Spectrometry (OES). The energy released in the various possible transitions can be depicted in the form of a line spectrum. Each element has its own individual line spectrum that is characteristic for this element.

Excited atoms are often chemically more reactive than atoms in the ground state and can participate in chemical processes in which they are not involved under normal circumstances.