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 Niels Bohr was born on October 7, 1885 into  a family who valued science and research. His father, [|Christian Bohr], was a professor of physiology, while his brother, [|Harald], became a mathematician.  Although faced with these high standards, Niels became successful in his own right. His first research project, completed in 1906, earned him a gold  medal from the Academy of Sciences. Later, b etween 1911 and 1912, he journeyed to England, studying at the [|University of Cambridge] under the well-known physicist [|J.J. Thomson] , and at the [|University of Manchester] under Ernest Rutherford . Under the tutelage of Rutherford, Bohr began to develop his later model of the atom, by first realizing that electrons orbit the nucleus. Taking knowledge from England, Bohr returned to Denmark and became a professor at the [|University of Copenhagen], where he had earned his Doctorate in 1911. Here, Bohr further developed his model of the atom. Danish authorities built the Institute of Theoretical Physics and appointed Bohr as director. Now the former pupil became a mentor to Wolfgang Pauli  and Werner Heisenberg , who would contribute to Bohr’s various works. In 1922, Bohr earned the most prestigious award of them all: [|the Nobel Prize for Physics]. His institute also discovered a new element, which they named [|hafnium] . Attention veered from quantum to nuclear physics in subsequent years and Bohr became a leading member of the field. He moved to the United States and participated in the [|Manhattan Project], helping develop the [|atomic bomb]. Later, however, Bohr became an opponent of nuclear warfare. In 1952, he assisted in creating the [|European Centre for Nuclear Research] <span style="font-family: Tahoma, Geneva, sans-serif;">in Switzerland, another <span style="font-size: 13pt; color: black; font-family: Tahoma; mso-fareast-font-family: 'Times New Roman'; mso-ansi-language: EN-US; mso-fareast-language: EN-US; mso-bidi-language: AR-SA;">achievement accomplished before his death on November 18, 1962.

<span style="font-size: 130%; font-family: Tahoma, Geneva, sans-serif;">Niels Bohr was a brilliant [|theoretical physicist], and was not afraid to abandon classical mechanics or to use unorthodox ideas in his reasoning. He first met Ernest Rutherford in [|J. J. Thompson's] laboratory at the [|University of Cambridge]—Bohr was quickly impressed by Rutherford's knowledge, vigor, and charm. They spoke at length about Max Plank, Albert Einstein, and the latest developments in physics, and Rutherford would soon become equally impressed with the young Bohr. He had a great energy and was invited to join Rutherford and his associates, an offer he took with excitement in 1912.



It was then that Bohr began to study some of the theoretical implications of Rutherford's atomic model and consider as to an explanation for its instability. Classical electromagnetic theory predicted that orbiting electrons—such as in Rutherford's planetary model of the atom—should emit [|radiation], and that this radiation should change in frequency as the electrons lose energy. If the electron were to steadily lose energy and thus spiral into the nucleus, who knows what would happen. Due to the fact that some atoms emit visible radiation that does not change color (which would indicate a frequency shift) over time and that the atom does not eventually explode, disappear, or otherwise destruct, it is obvious that there is [|something wrong] with Rutherford's atomic model. <span style="font-family: Tahoma, Geneva, sans-serif;">

<span style="font-size: 130%; font-family: Tahoma, Geneva, sans-serif;"><span style="font-family: Tahoma, Geneva, sans-serif;"> <span style="font-family: Tahoma, Geneva, sans-serif;">

Another, more direct issue with Rutherford's idea was his proposition that an electron's orbit could be found at any given distance from the nucleus. This was easily contradicted by the fact that the [|spectrum of light] emitted by atomic hydrogen is not a full spectrum but a [|series of lines]. This imposed some doubt upon the [|Aristotelian property of infinity] and invited confusion over the fact that in the Rutherford model, electrons had no set location, that the distance from the nucleus of the orbit of an electron was arbitrary.

<span style="font-family: Tahoma, Geneva, sans-serif;"> Taking a bold step away from classical physics, Bohr theorized that each electron could only exist in a discrete set of stable and stationary states--a limited set of distances from the nucleus. In the [|Bohr atomic model](see left), each state has a definite value of energy. This theory not only complemented the line spectrum of hydrogen (known as the [|Balmer series]) but expanded upon the ideas being introduced by other physicists such as Max Plank and Albert Einstein about [|quantum mechanics]. Bohr’s theory of a limited number of set states of electrons fit nicely with Plank’s idea that energy came in small packets of energy called [|quanta].



<span style="font-family: Tahoma, Geneva, sans-serif;"> The electrons that existed within these specific levels, according to Bohr's theory, moved between levels depending on their energy. An electron at energy level one, the [|ground state], could be moved to the second energy level if it acquired enough energy to do so, perhaps from [|cathode rays], [|X-rays], or even heat. If the energy is not enough for the electron to advance to energy level two (remember, it has to oppose the magnetic force of the nucleus), however, the electron will not be at an orbit //between// the two levels, it will remain at the [|ground state]. Therefore, electrons "jump" to a higher level of energy when they absorb energy and drop to a level of lower energy as they emit energy. The electron cannot exist between levels.

Bohr thus provided a more fitting explanation for [|electromagnetic radiation]. He introduced the idea that electrons didn't emit radiation when in a stable state, but rather while in a [|transition between states]. According to this theory, atoms did not absorb or emit [|radiation] continuously, but rather in finite, quantized steps. Bohr was applying the newfound theories of [|quantum mechanics] to the atom, and it was all making sense. Since <span style="font-family: Tahoma, Geneva, sans-serif;">transitions occurred between set states or levels, set and definite amounts of energy were involved. This would explain the reason why atoms give a [|line spectrum] of emitted energy (each line representing a transition from one energy state to another) and would also complement the idea that energy was [|quantum] in nature, that is was only emitted in finite bundles.

A lucid analogy developed by a contemporary professor was put forth at the time of the introduction of Bohr's theory to help explain it:

Niels Bohr <span style="font-size: 110%; font-family: Tahoma, Geneva, sans-serif;"><span style="font-family: Tahoma, Geneva, sans-serif;">  <span style="font-family: 'Arial','sans-serif';"><span style="font-family: Tahoma, Geneva, sans-serif;"><span style="font-size: 90%; font-family: Tahoma, Geneva, sans-serif;">  <span style="font-family: Verdana, Geneva, sans-serif;">  <span style="font-family: Tahoma, Geneva, sans-serif;"><span style="font-size: 120%; font-family: Tahoma, Geneva, sans-serif;">courageously made use of all of the information and theories of physics and science available to him at his time (no matter how unorthodox), sought to fulfill and successfully satisfied complementarity to an incredible extent, made great steps forward and provided key information as to the structure and behavior of the atom, and duly set the table for physicists after him to continue to research and explore the atom and to eventually put it in full adherence to quantum mechanics and the further development thereof.

<span style="display: block; font-size: 120%; font-family: Tahoma, Geneva, sans-serif; text-align: center;"> Here is a comprehensive video that should help with your understanding, courtesy of //Senior Physics//. <span style="display: block; font-family: Tahoma, Geneva, sans-serif; text-align: center;"> media type="youtube" key="bDUxygs7Za8" height="344" width="425" <span style="font-size: 120%; font-family: Tahoma, Geneva, sans-serif;"> //Click// <span style="font-size: 120%; font-family: Tahoma, Geneva, sans-serif;">[|here] //<span style="font-size: 120%; font-family: Tahoma, Geneva, sans-serif;"> for the next program. //

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 * For more information** on Niels Bohr and his work, feel free to visit the following websites:
 * http://nobelprize.org/nobel_prizes/physics/laureates/1922/bohr-bio.html
 * http://www.pbs.org/wgbh/aso/databank/entries/bpbohr.html
 * http://www.crystalinks.com/bohr.html
 * http://www.colorado.edu/physics/2000/index.pl

<span style="font-size: 200%; font-family: Tahoma, Geneva, sans-serif;">Bibliography

//Information// Abbott, David, ed. __Physicists__. New York: Peter Bedrick Books, 1984. Jaffe, Bernard. __Crucibles: The Story of Chemistry__. New York: Dover Publications, Inc., 1976. Senior Physics. __The Bohr Model__. Structure of the Atom. YouTube Video. <http://www.youtube.com/watch?v=hpKhjKrBn9s&annotation_id=annotation_736168&feature=iv>.
 * "Bohr atomic model."** Encyclopædia Britannica. __Ultimate Reference Suite__. Chicago: Encyclopædia Britannica, 2009.
 * "Bohr, Niels."** Encyclopædia Britannica. __Ultimate Reference Suite__. Chicago: Encyclopædia Britannica, 2009.

// Media (in the order in which it appears) //

http://www.mlahanas.de/Physics/Bios/images/NielsBohr1.jpg http://www.atomicarchive.com/Images/bio/B23.jpg http://upload.wikimedia.org/wikipedia/commons/d/d5/Niels_Bohr_Albert_Einstein_by_Ehrenfest.jpg http://astronomy.nmsu.edu/nicole/teaching/ASTR110/lectures/lecture18/slide01.html http://astronomy.nmsu.edu/nicole/teaching/ASTR110/lectures/lecture18/slide01.html http://csm01.csu.edu.tw/0299/tina_website/Learn/nature/Chemistry/oatoms0001a4.gif http://www.youtube.com/watch?v=hpKhjKrBn9s&annotation_id=annotation_736168&feature=iv

// Quotes // // (in the order in which they appear) // http://www.brainyquote.com/quotes/authors/n/niels_bohr.html http://www.brainyquote.com/quotes/authors/n/niels_bohr.html http://www.pbs.org/wgbh/aso/databank/entries/bpbohr.html Jaffe, Bernard. __Crucibles: The Story of Chemistry__. New York: Dover Publications, Inc., 1976. (244). Jaffe, Bernard. __Crucibles: The Story of Chemistry__. New York: Dover Publications, Inc., 1976. (244-245).

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