At this juncture, the following experimental observations require rationalization: the movement of electrons within the atom is quantized, while the kinetic energy of a free electron can take any value. Accordingly, Mott resolves this: "The answer to this is that quantization applies to any movement of particles within a confined space, or any periodic motion, but not to unconfined motion such as that of an electron moving in free space or deflected by a magnetic field. This is an experimental fact." To account for these experimental facts, empirical rules were introduced in the earliest days of quantum theory to enable the prediction of properties. The Bohr model of the hydrogen atom, appearing in 1913, successfully predicted energies but was fundamentally flawed¾giving an incorrect angular momentum of the ground state. This inconsistency resulted from his adoption of orbits to describe the observed energy quantization.