Classical physics In contrast to classical physics, "modern physics" is a slightly looser term which may refer to just quantum physics or to 20th and 21st century physics in general. Modern physics includes quantum theory and relativity, when applicable. A physical system can be described by classical physics when it satisfies conditions such that the laws of classical physics are approximately valid. In practice, physical objects ranging from those larger than atoms and molecules, to objects in the macroscopic and astronomical realm, can be well-described (understood) with classical mechanics. Beginning at the atomic level and lower, the laws of classical physics break
Electron affinity Electron affinity In chemistry and atomic physics, the electron affinity ("E") of an atom or molecule is defined as the amount of energy "released" or "spent" when an electron is added to a neutral atom or molecule in the gaseous state to form a negative ion. In solid state physics, the electron affinity for a surface is defined somewhat differently (see below). This property is measured for atoms and molecules in the gaseous state only, since in a solid or liquid state their energy levels would be changed by contact with other atoms or molecules. A list of the electron