By Robert M. Hazen, Ph.D., George Mason University
In order to understand the behavior of atoms, we must understand the Bohr atom. But, who was Niels Bohr, after whom the Bohr atom is named? And, why did the physicist embrace his strange and counterintuitive model?
Niels Bohr
Danish physicist Niels Bohr (1885–1962) was one of the most influential scientists to have ever lived. Bohr had earned a doctorate by his mid-20s and he’d also won a gold medal for his studies from the Danish Academy—a very distinguished early career.
In 1912, he joined Ernest Rutherford’s group at Manchester, where he began to work on the atomic theory.
Learn more about atoms.
Bohr’s Role in the Development of the Atom Bomb
During World War II, Bohr escaped to the United States and participated in the Manhattan Project that built the atomic bomb.
He was one of the very first people to express the view that the mere development of nuclear weapons might put an end to war altogether because it was so unthinkable that anyone would use nuclear weapons. Bohr was also a founding member of the Danish Atomic Energy Commission, which occurred just after the war.
Niels Bohr, a Celebrated Physicist
Niels Bohr was celebrated in Denmark and was awarded a mansion in Copenhagen. He was so admired in his day that there is a story around the special features of his mansion.
It is said that the mansion had a tap in the kitchen that was connected directly to the brewery, and on turning the tap, out came the beer. Undoubtedly, Bohr had achieved an unusual position of influence in the scientific community.
This is a transcript from the video series The Joy of Science. Watch it now, on Wondrium.
The Bohr Atom Model
It was while Bohr was working in England in 1913 that he developed this atomic model. He developed the model after studying the way glowing, hot hydrogen gives off light.
When an incandescent light bulb is lit, it gives off all the different wavelengths of light. As that filament heats up, all the different wavelengths come out of that light bulb because of the hot filament. Yet, when hydrogen gas is heated to a high temperature so that it becomes incandescent, something rather different is seen, a discrete wavelength—a line in the red of just one wavelength. There may be a line in the blue, in the yellow, and maybe some other lines, each one with a very specific wavelength.
Bohr suggested that this implies that electrons have to adopt very specific energy levels in an atom, rather than being at any possible level. These lines in glowing hydrogen represent times when electrons jump from a higher energy level to a lower energy level and, as a consequence, emit a photon of visible light at those very specific wavelengths. According to the first law of conservation, energy has to be conserved. So, the energy lost from jumping is emitted as a photon.
Learn more about the quantum world.
Allowed Orbits of the Bohr Atom
Electrons in the Bohr atom can reside in several different, discrete stationary states, which are rather inappropriately called allowed orbits. There is a nucleus, then an orbit at a well-defined distance, and another one, and another one in a kind of set of stair steps.
An electron can remain in one orbit for a long time without accelerating. It can rest in a stationary state and thus it doesn’t have to give off light or energy and so the atom becomes stable.
Ground and Excited States
The lowest energy level of the electron beyond which it cannot go is called the ground state. All the higher energy states are called excited states. As electrons hop from state to state, back and forth, specific wavelengths are either absorbed or emitted by that atom.
Learn more about the periodic table of the elements.
Drawing an Analogy
The Bohr atom lies completely outside any sort of natural experience that people have in their everyday life. In people’s lives, energy comes as a continuum but at the level of the atom there isn’t a continuous range, there are discrete steps.
One analogy that could make the Bohr atom a little more meaningful is to compare it with stair steps. When an object is placed stably on one step or another step, just as the different orbits of a Bohr atom have a different energy, so does the object on different steps. The higher the step, the more gravitational potential energy the object will have.
But unlike stair steps, electrons don’t move from one orbit to another, but rather the electron literally vanishes from one orbit and reappears in the next. So that’s quite counterintuitive.
Is the Bohr Atom a Miniature Solar System?
The Bohr atom is not like a miniature solar system. In a solar system, a planet can be at any distance from the sun and it would achieve a stable orbit. But in a Bohr atom, there are only very specific distances from the nucleus that the electron can occupy.
Furthermore, planets in their elliptical orbits are constantly accelerating. They’re accelerating because the force of gravity is constantly pulling them in toward the sun. But in the Bohr atom, the electrons are not accelerating and are in stable orbits.
Nonetheless, the Bohr atom was embraced by physicists because it was extremely successful in modeling the behavior of atoms, especially things like the way an atom interacted with light. The strangeness of the atom mirrors the strangeness of the world at this subatomic level and Bohr could successfully explain it. It was for all of this work on the atom that Niels Bohr won the 1922 Nobel Prize in physics, just one more to his many accolades.
Common Questions about Niels Bohr and the Bohr Atom Model
In 1912, Niels Bohr joined Ernest Rutherford’s group at Manchester, where he began to work on the atomic theory.
Niels Bohr was awarded the Nobel Prize in Physics in 1922 for his contribution to understanding the structure of the atom.
Electrons in the Bohr atom can reside in several different, discrete stationary states, which are called allowed orbits.
