Niels Bohr, at the University of Copenhagen, proposes in 1913 a model of the hydrogen atom that resolves a problem irreconcilable with classical physics: according to Maxwell's laws of electromagnetism, an electron orbiting a nucleus should radiate energy continuously and spiral into the nucleus within a fraction of a second, something that evidently does not happen in real matter. Bohr postulates, directly applying Planck's quantization, that electrons can only occupy specific, discrete orbits around the nucleus, each with a fixed energy; the electron does not radiate energy while remaining in a stable orbit, and only emits or absorbs energy — in the form of a photon, per Einstein's concept — when jumping between two discrete levels. This model, although it would be superseded years later by the complete quantum mechanics of Heisenberg and Schrödinger, explains with extraordinary precision the line spectrum of the hydrogen atom, a phenomenon observed experimentally decades earlier without a satisfactory theoretical explanation. The Bohr model establishes the central idea that energy within the atom is quantized into discrete levels — a concept that survives throughout later atomic physics and quantum chemistry, even after abandoning the literal image of planetary "orbits".