High Quality Content by WIKIPEDIA articles! Quantum mechanics (QM) is a set of scientific principles describing the known behavior of energy and matter that predominate at the atomic and subatomic scales. The name derives from the observation that some physical quantities such as the energy of an electron can be changed only by set amounts, or quanta, rather than being capable of varying by any amount. The wave particle duality of energy and matter at the atomic scale provides a unified view of the behavior of particles such as photons and electrons. Photons are the quanta of light, and have energy values proportional to their frequency via the Planck constant. An electron bound in an atomic orbital has quantized values of angular momentum and energy. The unbound electron does not exhibit quantized energy levels, but is associated with a quantum mechanical wavelength, as are all massive particles. The full significance of the Planck constant is expressed in physics through the abstract mathematical notion of action.