Niels Bohr came from a very distinguished scientific family in Copenhagen, Denmark. His father, Christian, was professor of physiology at Copenhagen and his brother Harald was a mathematician of great distinction. (His own son, Aage, was later to win the 1975 Nobel Prize for physics.) Bohr was educated at the University of Copenhagen where he obtained his PhD in 1911. After four productive years with Ernest Rutherford in Manchester, Bohr returned to Denmark becoming in 1918 director of the newly created Institute of Theoretical Physics.

Under Bohr (who after Albert Einstein was probably the most respected theoretical physicist of the century) the institute became one of the most exciting research centers in the world. A generation of physicists from around the world were to pass through it and eventually it was to bestow on the orthodox account of quantum theory the apt description of the ‘Copenhagen interpretation’.

In 1913 Bohr published a classic paper, On the Constitution of Atoms and Molecules, in which he used the quantum of energy, h, introduced into physics by Max Planck in 1900, to rescue Rutherford's account of atomic structure from a vital objection and also to account for the line spectrum of hydrogen. The first problem Bohr faced was to explain the stability of the atom. Rutherford's 1911 model of the atom with electrons orbiting a central nucleus (the so-called planetary model) was theoretically unstable. This was because, unlike planets orbiting the Sun, electrons are charged particles, which, according to classical physics, should radiate energy and consequently spiral in toward the nucleus.

Bohr began by assuming that there were ‘stationary’ orbits for the electrons in which the electron did not radiate energy. He further assumed that such orbits occurred when the electron had definite values of angular momentum, specifically values h/2π, 2h/2π, 3h/2π, etc., where h is Planck's constant. Using this idea he was able to calculate energies E ₁, E ₂, E ₃, etc., for possible orbits of the electron. He further postulated that emission of light occurred when an electron moved from one orbit to a lower-energy orbit; absorption was accompanied by a change to a higher-energy orbit. In each case the energy difference produced radiation of energy hν, where ν is the frequency. In 1913 he realized that, using this idea, he could obtain a theoretical formula similar to the empirical formula of Johannes Balmer for a series of lines in the hydrogen spectrum. Bohr received the Nobel Prize for physics for this work in 1922. The Bohr theory was developed further by Arnold Sommerfeld.

Bohr also made other major contributions to this early development of quantum theory. The ‘correspondence principle’ (1916) is his principle that the quantum-theory description of the atom corresponds to classical physics at large magnitudes.

In 1927, Bohr publicly formulated the ‘complementarity principle’. This argued against continuing attempts to eliminate such supposed difficulties as the wave–particle duality of light and many other atomic phenomena. His starting point was the impossibility to distinguish satisfactorily between the actual behavior of atomic objects, and their interaction with the measuring instruments that serve to define the conditions under which the phenomena appear. Examine light with one instrument, the argument went, and it undulates like a wave; select another and it scatters like a particle. His conclusion was that evidence obtained under different experimental conditions cannot be comprehended within a single picture, but must be regarded as complementary in the sense that only the totality of the phenomenon exhausts the possible information about the objects. It was a principle Bohr remained faithful to, even representing it on his coat of arms in 1947 with the motto Contraria sunt complementa above the Yin/Yang symbols. Together with the indeterminancy principle of Werner Heisenberg and the probability waves of Max Born, this principle emerged from the 1930 Solvay conference (the last one Einstein attended) as the most authoritative and widely accepted theory to describe atomic phenomena.

Bohr also made major contributions to the work on radioactivity that led to the discovery and exploitation of nuclear fission. Bohr's liquid-drop model of the nucleus, which was published in 1936, provided the basis for the first theoretical account of fission worked out in collaboration with John Wheeler in 1939. It was also Bohr who, in 1939, made the crucial suggestion that fission was more likely to occur with the rarer isotope uranium–235 than the more common variety uranium–238.

In 1943 Bohr, who had a Jewish mother, felt it necessary to escape from occupied Denmark and eventually made his way to Los Alamos in America where he served as a consultant on the atomic bomb project. He was quick to appreciate the consequences of using such weapons and in 1944 made an early approach to Roosevelt and Churchill proposing that such obvious danger could perhaps be used to bring about a rapprochement between Russia and the West. Scientists were in a unique position, he argued, in having the Soviet contacts and the knowledge to make the first approach. Much of Bohr's time after the war was spent working, among scientists, for adequate controls of nuclear weapons and in 1955 he organized the first Atoms for Peace conference in Geneva.