September 15, 2021   Read time 5 min
The Danish physicist Niels Bohr created the framework for modern atomic physics when he brilliantly combined Ernest Rutherford’s model of the nuclear atom with Max Planck’s quantum theory and then assigned specific, nonradiating orbits to the atomic electrons.

Niels Henrik David Bohr was born on October 7, 1885, in Copenhagen, Denmark. He came from a very distinguished and accomplished family that provided a nurturing environment for his genius. His father, Christian Bohr, was a professor of physiology at the University of Copenhagen. His mother, Ellen née Adler, came from a wealthy Jewish family that was prominent in the field of education. His younger brother, Harald, was a very skilled mathematician and an Olympic medalist (1908 silver, with the Danish soccer team). Bohr’s own son, Aage, would win the Nobel Prize for physics in 1975.

Bohr entered Copenhagen University in 1903 and earned his M.S. degree in physics in 1909 and his Ph.D. in physics in 1911. In the spring of 1912, Bohr joined Ernest Rutherford (1871–1937) at his laboratory in Manchester, England, where he studied radioactivity and atomic theory. Rutherford had just introduced his nuclear atomic model. Breaking with the “plum pudding” model of the British scientist J. J. Thomson, Rutherford’s model had the electrons moving in circular orbits around a positively charged central nucleus. However, Rutherford’s model contained some fundamental difficulties. Contradictory to prevailing electromagnetic theory, the electrons in the Rutherford atom did not emit electromagnetic radiation.

In 1913, Bohr proposed a brilliant modification of Rutherford’s atomic nucleus hypothesis. Bohr incorporated Planck’s quantum model and postulated that there were discrete energy levels (or shells) occupied by the orbiting electrons within which they could move but did not emit electromagnetic radiation. Only if these orbiting electrons were raised to a higher energy level or dropped to a lower energy level would they either absorb or emit electromagnetic radiation.

The Bohr atom, as this model is now known, resolved problems with the Rutherford atom and also cleverly explained the observed spectral lines of the hydrogen atom. Bohr summarized these thoughts in his classic 1913 paper “On the Constitution of Atoms and Molecules.” He had applied Planck’s quantum theory and resolved the theoretical instability of Rutherford’s atomic model. In less than a decade, physicists around the world recognized how Bohr’s work significantly refined the structure of the atom. He received the 1922 Nobel Prize in physics for this great accomplishment.

From 1913 to 1914, Bohr held a lectureship in physics at Copenhagen University. He then held a similar appointment at Victoria University in Manchester, England, from 1914 to 1916. He received an appointment as professor of theoretical physics at Copenhagen University in 1916. Recognizing Bohr’s brilliance as a scientist, officials at the university created the Institute of Theoretical Physics exclusively for him in 1920, with generous sponsorship and support from the Carlsberg brewery. Bohr was appointed director of this institute and he retained that position for the rest of his life.

Under Bohr—who was probably the most respected theoretical physicist of the twentieth century save Albert Einstein (1879–1955)—the Copenhagen Institute became one of the most exciting theoretical research centers in the world. An entire generation of young physicists would pass through Bohr’s institute and there openly discuss, challenge, and further refine contemporary atomic theory. The exciting wave of changes in quantum mechanics in the 1920s encouraged Bohr to propose his famous “concept of complementarity”—that things may have a dual nature, but we can experience only one aspect at a time. The wave and particle dual nature of the electron is an example.

In the 1930s, Bohr focused the activities of his institute more and more on the atomic nucleus and the interesting transmutation processes and disintegrations that were being reported by many experimental physicists. He made major theoretical contributions that led to the discovery and exploitation of nuclear fission. In particular, Bohr’s liquid-drop model of the nucleus, introduced in 1936, provided the starting point for Lise Meitner and Otto Frisch to suggest in early 1939 that Otto Hahn and Fritz Strassmann had split the atomic nucleus of a uranium-235 atom in Hahn’s Berlin laboratory late in 1938. Bohr heralded the exciting possibilities in his own short note “Disintegration of Heavy Nuclei,” published in the February 25, 1939, issue of Nature. Bohr also played an important role in early 1939 by personally carrying this startling news to physicists in the United States.

In 1943, worsening conditions in German-occupied Denmark forced Bohr, whose mother was Jewish, to flee with his family. They were hidden in a fishing boat and taken to Sweden by Danish resistance fighters. Once his family was safe in Sweden, Bohr and his son Aage were whisked away to England in the empty, unpressurized bomb bay of the British military aircraft that supported the secret, commando-like, operation. Bohr almost suffocated during the journey, but he recovered and soon accompanied the British government’s team of scientists working on the atomic bomb in Los Alamos, New Mexico.

A gentle, sensitive human being, Bohr quickly appreciated the global consequences of developing and using powerful nuclear weapons. So in 1944, he began to lobby Allied leaders, including U.S. president Franklin D. Roosevelt and British prime minister Winston Churchill, to consider implementing a postwar nuclear arms control strategy that included the Soviet Union. Although his nuclear peace initiatives did not achieve their desired effect, Bohr continued to provide theoretical physics support to the Manhattan Project.

Bohr returned to his institute in Copenhagen after World War II and then spent much of his time promoting adequate controls of nuclear weapons and the use of nuclear energy for peace. He was instrumental in the founding of CERN (the European Organization for Nuclear Research) in 1952 near Geneva, Switzerland. He also helped to organize the 1955 Atoms for Peace Conference in Geneva. He died at home in Copenhagen on November 18, 1962, following a stroke. Element 107, discovered by scientists at the Gellschaft für Schwerionenforschung (GSI) in Darmstadt, Germany, in 1981, was named bohrium (Bh) in recognition of Bohr’s lasting contributions to nuclear physics.