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Niels Bohr: Biography & Atomic Theory

Niels Bohr
Left: Niels Bohr in 1922. Right: A 1963 Danish stamp honored Bohr on the 50th anniversary of his atomic theory.
Credit: Left: AB Lagrelius & Westphal, via American Institute of Physics. Right: Antonio Abrignani / Shutterstock.com

Niels Bohr was one of the foremost scientists of modern physics, best known for his substantial contributions to quantum theory and his Nobel Prize-winning research on the structure of atoms.

Born in Copenhagen in 1885 to well-educated parents, Bohr became interested in physics at a young age. He studied the subject throughout his undergraduate and graduate years and earned a doctorate in physics in 1911 from Copenhagen University.

It was while conducting research for his doctoral thesis on the electron theory of metals that Bohr first came across Max Planck's early quantum theory, which described energy as tiny particles, or quanta.

In 1912, Bohr was working for the Nobel laureate J.J. Thompson in England when he was introduced to Ernest Rutherford, whose discovery of the nucleus and development of an atomic model had earned him a Nobel Prize in chemistry in 1908. Under Rutherford's tutelage, Bohr began studying the properties of atoms.

Combining Rutherford's description of the nucleus and Planck's theory about quanta, Bohr explained what happens inside an atom and developed a picture of atomic structure. This work earned him a Nobel Prize of his own in 1922.

In the same year that he began his studies with Rutherford, Bohr married the love of his life, Margaret Nørlund, with whom he had six sons. Later in life, he became president of the Royal Danish Academy of Sciences, as well as a member of scientific academies all over the world.

Atomic model

Bohr's greatest contribution to modern physics was the atomic model. The Bohr model shows the atom as a small, positively charged nucleus surrounded by orbiting electrons.  

lithium atom
A stylized representation of a lithium atom illustrates Niels Bohr's atomic model, that an atom is a small, positively charged nucleus surrounded by orbiting electrons.
Credit: Boris15 | Shutterstock

Bohr was the first to discover that electrons travel in separate orbits around the nucleus and that the number of electrons in the outer orbit determines the properties of an element.

The chemical element bohrium (Bh), No. 107 on the Periodic Table of the Elements, is named for him.

Liquid droplet theory

Bohr's theoretical work contributed significantly to scientists' understanding of nuclear fission. According to his liquid droplet theory, a liquid drop provides an accurate representation of an atom's nucleus.

This theory was instrumental in the first attempts to split uranium atoms in the 1930s, an important step in the development of the atomic bomb.

Despite his contributions to the U.S. Atomic Energy Project during World War II, Bohr was an outspoken advocate for the peaceful application of atomic physics.

Quantum theory

Bohr's concept of complementarity, which he wrote about in a number of essays between 1933 and 1962, states that an electron can be viewed in two ways, either as a particle or as a wave, but never both at the same time.

This concept, which forms the basis of early quantum theory, also explains that regardless of how one views an electron, all understanding of its properties must be rooted in empirical measurement. Bohr's theory stresses the point that an experiment's results are deeply affected by the measurement tools used to carry them out.

Bohr's contributions to the study of quantum mechanics are forever memorialized at the Institute for Theoretical Physics at Copenhagen University, which he helped found in 1920 and headed until his death in 1962. It has since been renamed the Niels Bohr Institute in his honor.

Niels Bohr quotations

"Every great and deep difficulty bears in itself its own solution. It forces us to change our thinking in order to find it."

"Everything we call real is made of things that cannot be regarded as real."

"The best weapon of a dictatorship is secrecy, but the best weapon of a democracy should be the weapon of openness."

"Never express yourself more clearly than you are able to think."

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