Niels Bohr

Danish physicist

• Born: October 7, 1885; Copenhagen, Denmark
• Died: November 18, 1962; Copenhagen, Denmark

Twentieth-century Danish physicist Niels Bohr received the Nobel Prize in Physics in 1922 for his work on creating a model of atomic structure. His theory that electrons orbit an atom’s nucleus formed the foundation of quantum mechanics. The chemical element bohrium is named for him.

Primary field: Physics

Specialty: Quantum mechanics

Early Life

Niels Henrik David Bohr was born in Copenhagen on October 7, 1885. His father, Christian Bohr, was a renowned professor, physiologist, and Nobel Prize nominee. His mother was from a family of educators. He was an accomplished student, one of his boyhood projects winning him a gold medal from the Royal Danish Academy of Sciences in Copenhagen.

In 1903, Bohr entered the University of Copenhagen. He studied under physicist Christian Christiansen, earning his master’s degree in physics in 1909 and his PhD in 1911. While writing his thesis on the electron theory of metals, he first came across German physicist Max Planck’s quantum theory, in which he described energy as small parcels, or quanta.

Bohr then traveled to England to begin his postdoctoral work for physicist J. J. Thomson, who won the Nobel Prize in 1906 for his discovery of the electron. During his time there, Bohr began a four-year association with Lord Ernest Rutherford, who had discovered the nucleus and developed a model of the atom. Under Rutherford, Bohr studied the properties of atoms.

In 1920, Bohr became the head of the Institute for Theoretical Physics at the University of Copenhagen (later renamed the Niels Bohr Institute). At the institute, he explored the structure of atomic nuclei and the changes that take place within them. During this time, Copenhagen became a world center for the study of quantum mechanics.

Bohr married Margrethe Nørlund in 1912. One of their six sons, Aage Bohr, followed his father as director of the Institute for Theoretical Physics and himself won a Nobel Prize in 1975.

Life’s Work

During Bohr’s association with Rutherford, he combined the British physicist’s description of the nucleus—Rutherford suggested that the atom has a miniature, dense nucleus surrounded by electrons—with Planck’s early ideas about quantum theory and explained what happens inside an atom. Planck had found that the energy radiated from a heated body is exactly proportional to the wavelength of its radiation; the wavelength equals the energy times a number called Planck’s constant.

Bohr also developed a picture of atomic structure. In fact, Bohr’s theory that electrons orbit an atom’s nucleus helped form the foundation of quantum mechanics. The Bohr model shows the atom as a small, positively charged nucleus surrounded by orbiting electrons. Bohr discovered that electrons travel in separate orbits around the atom’s nucleus, and that the number of electrons in the outer orbits determines the properties of the elements. He established that an electron could drop from a higher-energy orbit to a lower one, giving off energy in the process.

Bohr targeted the simplest atom of all: hydrogen. Hydrogen atoms radiate at certain frequencies, and Bohr looked for a formula to explain this. He found that an electron dropping from one energy level to another created the frequency. The difference between the electron’s beginning and ending energies was released as quantum energy. His discovery accurately explained the physical and chemical properties of the elements.

According to Bohr’s model, electrons existed at set levels of energy and fixed distances from the nucleus. If the atom absorbed energy, the electron jumped to a level further from the nucleus; if it gave off energy, it fell to a level closer to the nucleus. This solution explained the series of lines observed in the spectrum of light emitted by atomic hydrogen. Bohr was able to predict the frequencies of these spectral lines using the charge and mass of the electron and Planck’s constant. Bohr also proposed that an atom would not emit radiation while it was in one of its stable states, but only when it changed states. The frequency of the radiation given off would be equal to the difference in energy between those states divided by Planck’s constant. This meant that the atom could neither absorb nor emit radiation continuously, but only in quantum jumps.

Beginning in the 1920s, Bohr began a long-term association with Albert Einstein, and the two often discussed quantum theory. Einstein did not agree with Bohr about exactly how quantum mechanics operated, and the two men debated the meaning of quantum theory for the rest of their lives. Through the early 1920s, Bohr also concentrated his efforts on two interrelated sets of problems: He tried to develop a consistent quantum theory that would replace classical mechanics and electrodynamics at the atomic level. He also tried to explain the structure and properties of the atoms of all the chemical elements.

In 1936, Bohr developed the liquid droplet theory, which said that a liquid drop could give an accurate representation of an atom’s nucleus. Bohr’s liquid drop model treats the nucleus as a drop of nuclear fluid. Because nuclei seem to have almost constant density, the nuclear radius can be calculated by using that density in the same way that one would if the nucleus were a drop of a uniform liquid. This model also explains the spherical shape of most nuclei and helps predict the results of nuclear fission by comparing the energy necessary to change the shape of the drop to the energy added by a neutron joining the nucleon. When other scientists split the uranium atom three years later, the droplet theory helped explain nuclear fission: If the energy is great enough, the drop breaks apart, resulting in fission.

Bohr also contributed greatly to the field of quantum physics, particularly his development of the concept of complementarity. Bohr’s principle of complementarity says that an electron can be viewed two ways, either as a particle or as a wave, but never as both at the same time, and which it is depends entirely on the method of measurement. According to Bohr, at the atomic level, objects are unavoidably affected by being measured, and so guesses beyond measurement cannot be proven. For example, the question, “Where was the particle before I measured its position?” cannot be answered.

In 1939, Bohr visited the United States with the news that German scientists were working on splitting the atom. This prompted the United States to launch the Manhattan Project to develop an atomic bomb. Following Germany’s occupation of Denmark during World War II, Bohr learned that he was to be arrested by the Nazis; he escaped, residing in England and the United States for the remainder of the war. During this period, he contributed to the development of the first atomic bomb. Even while working on the first atomic bomb, Bohr worried about the danger and misuse of nuclear weapons after the war. He felt that if countries shared their research and avoided secrecy, many of the political and ethical problems associated with nuclear energy could be avoided.

After the war, Bohr returned to Copenhagen and continued his advocacy regarding the peaceful use of nuclear technology. He died in Copenhagen in 1962.

Impact

One part of Bohr’s theory, called the correspondence principle, became especially important in the overall development of quantum theory, which in turn shaped all of modern physics. According to the correspondence principle, the results of quantum mechanics do not conflict with those of classical mechanics in the realm of physical phenomena, where classical laws are valid. Bohr’s original theory was therefore extended to other areas of physics.

Bohr’s theory was able to make remarkably accurate predictions for atoms with a single electron; it was less reliable, however, when it was applied to atoms with more than one electron. In 1920, Bohr focused on this problem and presented an improved and consistent theory.

Bohr’s groundbreaking atomic model of 1913, although flawed, paved the way for quantum mechanics, which would ultimately dominate twentieth-century physics. Bohr himself not only continued to contribute to physics but also educated a new generation of physicists who went on to develop quantum mechanics. Although he never became as famous as Albert Einstein, his work is among the most important in the history of physics.

Bibliography

Kumar, Manjit. Quantum: Einstein, Bohr, and the Great Debate About the Nature of Reality. New York: Norton, 2008. Print. Discusses the roles of Bohr and Albert Einstein in the development of quantum theory and the historical and scientific implications of their debate.

Lindley, David. Uncertainty: Einstein, Heisenberg, Bohr, and the Struggle for the Soul of Science. New York: First Anchor, 2007. Print. Presents an overview of modern physics and quantum mechanics in the twentieth century from a social and historical perspective, also discussing the work of Bohr and scientists including Werner Heisenberg and Albert Einstein.

Ottaviani, Jim, and Leland Purvis. Suspended in Language: Niels Bohr’s Life, Discoveries, and the Century He Shaped. Ann Arbor: G.T. Labs, 2004. Print. Illustrated science biography of Bohr’s life and work with discussion of quantum mechanics, physics, and history.