Niels Henrik David Bohr was a Danish physicist born on Oct 7, 1885 in Copenhagen, Denmark. He was raised a Christian by his mother who was Jewish, and he died as a Lutheran. Although his accomplishments are numerous, he is best known for an early model of the atom and is considered the father of the Quantum Theory. Bohr died of a stroke in Nov 18, 1962.
Bohr is known for winning multiple accolades; the Nobel Prize for Physics in 1922, Hughes medal in 1921, Matteucci medal in 1923, Copley medal in 1938, and the Sonning Prize in 1961. Bohr went to Gammelholm Grammar School, Copenhagen for high school in 1903. He received both his MA and PhD from the University of Copenhagen in both 1909 and 1911 consecutively. Bohr became a Lecturer in his University and a Lecturer in the Victoria University of Manchester in 1913 - 1916. Bohr is considered to be the Father of Quantum Theory. Along with Ernest Rutherford Bohr created an atomic model that proved that electrons traveled in orbits around the atoms nucleus and would go on to explain how electrons would absorb and or emit energy. Bohr's atomic model won him the Nobel Prize in Physics in the year of 1922. Bohr is well known for two specific principles, which were the Complementarity principle and the Correspondence principle.
Bohr's Atomic Model
While working in Rutherford's laboratory Bohr took it upon himself to answer questions concerning electrons, the arrangement of electrons, and whether they were moving or not. Bohr knew that the nucleus had a positive charge and that electrons had negative charges. What he did not understand was why the electrons would not fall inside the nucleus. It was an assumption as the earth's momentum balances the gravitation attraction of the sun that the electrons would move around the nucleus and that their momentum would keep them from falling into the nucleus. Spectroscopy was a major support for Bohr as he devised the model that described the movement of electrons around the nucleus (Each element contains a set of colored lines or a line spectrum to be exact).
In 1913, while working on hydrogen, Bohr devised an atomic model that explained these bright lines that were on scientists mind's for what would seem many decades. Bohr concluded that electrons would only exist through definite energy levels outside the nucleus, energy levels near the nucleus would correspond to lower levels of potential energy, and levels at great distances would correspond to high potential energy levels. Bohr also concluded that the movement of electrons in the various levels of energy would explain the bright-line spectra of elements that were gaseous. Bohr said that when the right amount of energy and intensity of electricity, heat and light, would excite an atom the electrons would then jump up to a higher energy level. The change in energy levels had a definite quantify (quantized).
Bohr went on to say that the atom would not stop between levels and that once the atom was at its highest energy level it would return to its lowest energy state as soon as possible. Bohr would say that the potential energy the electrons contained at the high level would convert to light/electromagnetic energy when it returned to the lower level. The colors that were observed in the spectra gave important clues about the energy levels in atoms. Bohr was astonishingly able to predict the exact wave lengths of the lines in the hydrogen spectrum. Bohr's energy levels were referred to as principal energy levels. In theory there are many principal energy levels although only six or seven can be measured. 
The Bohr model contained four principles. The first had to do with how electrons would only assume specific orbits that would go around the nucleus. The second had to do with how an individual orbit would have an energy related to it; the orbit that was nearest to the nucleus would have an energy of E1, and the one that was closest to that obit would have E2, "increasing as it continued on". Third, light can only be emitted when the electron jumps with a reaction then is absorbed when the higher orbit falls to a lower orbit soon after the lower orbit jumps to the higher orbit. The fourth principle can be seen through an equation which is, and applies to principle three, E(light) = Ef - Ei n = E(light)/h h= Planck's constant = 6.627x10-34 Js. Through these principles many of the questions over the stability of atoms and Hydrogen's emission spectrum can be answered. 
Although Bohr's model has been viewed as highly successful it does have its faults; Bohr's model considers electrons to have a known orbit and radius it clearly violates Heisenberg's Uncertainty principle. The Bohr model provided an inaccurate value of the ground state and orbital angular momentum. The Bohr model does not explain the Zeeman effect which is "the dividing of a spectral line or lines as a result of placing a radiation source in a magnetic field." (dictionary.com) Another fault that can be considered is the fact that the Bohr model does not expound on both fine structure and hyperfine structure, which is apart of the Spectra lines. Along with the fine structure and hyperfine structure, the intensities in spectra lines are not really shown in Bohr's model. 
Bohr's Correspondence Principle
Bohr's Correspondence Principle is known to be immensely undisputed. Although this is known to be true there are still some scholars that argue on how the correspondence principle should be defined taking to fact that there are several relations between the classic and quantum mechanic's that Bohr discovered. There are known to be three primary definitions of literature in Bohr's writings.
According to the Stanford Encyclopedia of Philosophy the first is the frequency interpretation which states that:
|“||the correspondence principle is a statistical asymptotic agreement between one component in the Fourier decomposition of the classical frequency and the quantum frequency in the limit of large quantum numbers.||”|
The second is the intensity interpretation which states that:
|“||it is a statistical agreement in the limit of large quantum numbers between the quantum intensity, understood in terms of the probability of a quantum transition, and the classical intensity, understood as the square of the amplitude of one component of the classical motion.||”|
The last definition is the selection rule interpretation which states that:
|“||the correspondence principle is the statement that each allowed quantum transition between stationary states corresponds to one harmonic component of the classical motion.||”|
Bohr's Complementarity Principle
The Complementarity Principle was originally introduced in Bohr's 1927 Como Lecture. Bohr saw the need for mathematical formalism for quantum physics to be a framework of an acceptable scientific theory. Bohr explained in his Complementarity Principle that "the impossibility of any sharp separation between the behavior of atomic objects and the interaction with the measuring instruments which serve to define the conditions under which the phenomena appear." Bohr went on to say that "evidence obtained under different experimental conditions cannot be comprehended within a single picture, but must be regarded as complementary in the sense that only the totality of the phenomena exhausts the possible information about the objects." Bohr's Principle was admired by quite a few Physicist one in particular was Einstein. Einstein referred to Bohr's early work as "the highest form of musicality in the sphere of thought." Although Einstein accepted much of Bohr's early work, he did not agree with all his concepts. In particular, Einstein did not accept Bohr's idea that quantum mechanics should be defined as "rational generalization of classical physics." 
His father Christian Bohr, was a teacher in physiology. His mother was Ellen Adler Bohr. Bohr had both a brother and a sister, his sister was Jennifer known to Bohr and his family as "Jenny" his brother was Harald August Bohr, Herald was a mathematician. Bohr was married to wife Margrethe Norlund and had three sons. Aage Bohr, who followed in his fathers footsteps, became a physicist and won a Nobel prize; Erik Bohr turned out to be a chemist; Ernest Bohr became an attorney; and Hans Henrik Bohr became a physician.
Niels Bohr discussing with Max Planck
- Niels Bohr Copyright ©2011 Soylent Communications.
- Cox, Porch, and Wetzel. Chemistry for Christian Schools. South Carolina: Bob Jones University Press, 2000.(p.73-74).
- Bohr model of the atom Professor N. De Leon , Spring 2001.
- Bohr Model Anne Helmenstine, Ph.D, About.com Guide, New York Times Company.
- Bohr's Correspondence Principle Bokulich, Alisa, Thu Oct 14, 2010.
- [www-physics.lbl.gov/~stapp/Complementarity.doc Complementarity Principle] Bohr, N. (1934): Atomic theory and the description of nature. Cambridge University Press, Cambridge UK, Bohr, N. (1963): Essays 1958-1962 on Atomic Physics and Human Knowledge. Wiley, New York.
- Bohr, Niels Copyright © 1997 Encyclopædia Britannica, Inc. All Rights Reserved, This article was written in part by Martin J. Klein, who is Eugene Higgins Professor of Physics and the History of Science at Yale University.