Quantum mechanics is a set of principles which underlies fundamental known description of all physical systems at the microscopic scale at the atomic level. It is the study of matter and radiation at an atomic level. Among these principles are both a dual wave-like and particle-like behavior of matter and radiation, and prediction of probabilities in situations where classical physics predicts certainties. Classical physics can be derived as a good approximation to quantum physics, typically in circumstances with large numbers of particles. Thus quantum phenomena are relevant in systems whose dimensions are close to the atomic scale, such as molecules, atoms, electrons, protons and other subatomic particles.
The word quantum refers to a discrete unit that quantum theory assigns to certain physical quantities, such as the energy of an atom at rest. Quantum mechanics is vital in order to comprehend the behavior of systems at atomic length scales. If classical mechanics governed the workings of an atom, electrons would rapidly travel towards and collide with the nucleus, making stable atoms impossible. Nevertheless, in the natural world the electrons usually remain in an unknown orbital path around the nucleus, defying classical electromagnetism.