American-Danish physicist who shared the 1975 Nobel Prize for Physics with Aage N. Bohr (Bohr, Aage N.) and James Rainwater (Rainwater, James) for his work in determining the asymmetrical shapes of certain atomic nuclei and the reasons behind such asymmetries.

Having taken his doctorate in theoretical physics at Harvard University in 1950, Mottelson accepted a fellowship at the Niels Bohr Institute of Theoretical Physics in Copenhagen and then joined the faculty of the Nordic Institute for Theoretical Nuclear Physics there; he subsequently became a naturalized Danish citizen.

From experiments conducted in collaboration with Bohr in the early 1950s, Mottelson discovered that the motion of subatomic particles can distort the shape of the nucleus, thus challenging the widely accepted theory that all nuclei are perfectly spherical. Subsequently it was discovered that such asymmetries occur in atoms of all elements.