Niels Henrik David 1885–1962
Danish physicist who investigated atomic structure and radiation. Bohr discovered that electrons orbit the nucleus of an atom at set distances, changing levels only when energy is lost or gained and emitting or absorbing radiation in the process. His concepts were fundamental to the later development of quantum mechanics.
Biography In 1922 Danish physicist Niels Bohr was awarded the Nobel Prize for physics for his ability to build upon the findings of Ernest Rutherford and develop a theory of atomic structure that would contribute significantly to the development of quantum mechanics. At the beginning of the twentieth century, before Bohr's discovery, scientists thought that atoms were a loosely combined mixture of electrons, protons, and neutrons. In 1911 Ernest Rutherford discovered that atoms had an extremely small, positively charged nucleus that contained no electrons, and he developed an atomic model that resembled the solar system, with negatively charged electrons orbiting a central nucleus. Rutherford's model was considered puzzling because it predicted that atoms should be unstable: since the electrons were orbiting the nucleus, they were undergoing acceleration, but accelerating electric charges give off electromagnetic energy, so the orbiting electrons should have been constantly giving off energy, and ultimately spiraling into the nucleus. But electrons did not do this. To explain the atom's apparent stability, Bohr postulated that electrons travel only in discrete orbits of different sizes and energy levels around the nucleus, and that increases or decreases in an electron's energy cause it to jump to a higher or lower orbit, absorbing or emitting energy in the form of electromagnetic radiation. Bohr's model explained why hydrogen, the simplest atom, emits and absorbs light only of certain frequencies depending on the difference in energy levels of the orbits between which the electron moves. Later in his career, Bohr developed the concept of complementarity to encompass wave-particle duality, the phenomenon that under some conditions light exhibits wavelike behavior and under other conditions particlelike behavior.