In 1913 he drew the atom the world still pictures — electrons orbiting like planets.
Niels Bohr (1885–1962) was the Danish physicist whose 1913 model of the atom — electrons circling the nucleus in fixed shells — explained the structure of matter, opened the first era of quantum theory, and won him the 1922 Nobel Prize in Physics.
Founder of the Copenhagen institute that became the world capital of quantum physics, he spent the 1920s in famous debate with Einstein over what quantum mechanics means — and the 1940s helping, then warning, the atomic age he had made possible.
Bohr was born in 1885 in Copenhagen, son of a physiology professor, and took his doctorate on the electron theory of metals. In 1912 he joined Ernest Rutherford in Manchester, just after Rutherford had discovered the atomic nucleus — a tiny, dense center with electrons around it, which classical physics said should instantly collapse.
Bohr's leap was to apply Max Planck's quantum idea: electrons could occupy only certain orbits, jumping between them by absorbing or emitting fixed packets of energy. The model explained hydrogen's spectral lines exactly.
In three papers published in 1913, Bohr laid out the shells — electrons arranged in orbits around the nucleus, each shell holding a fixed number. Chemistry's periodic table suddenly had a physical explanation: elements in the same column behave alike because their outer shells match.
In 1921 he founded the Institute for Theoretical Physics in Copenhagen, which drew Heisenberg, Pauli, Dirac and a generation of quantum pioneers. His principle of complementarity — that light is both wave and particle depending on how you measure it — became the philosophical heart of the Copenhagen interpretation.
Through the late 1920s Bohr debated Einstein, who insisted 'God does not play dice' against quantum randomness; Bohr answered that Einstein should stop telling God what to do. When the Nazis occupied Denmark, Bohr — half-Jewish — was smuggled to Sweden in a fishing boat in 1943, then to Britain and America, where he consulted on the Manhattan Project.
After the war he campaigned for openness about atomic energy and international control — 'atoms for peace' before the phrase existed. His institute, now the Niels Bohr Institute, remains a world center of physics; element 107, bohrium, is named for him, and his son Aage shared the 1975 Nobel Prize in Physics.
