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Niels Bohr Atomic Theory


The quantum theory of atomic processes was proposed by Niels Bohr in the year 1913, and was based on the atomic model proposed earlier by Rutherford. The deduction of the Balmer series for the line spectrum of hydrogen and of the Rydberg number from universal atomic constants constituted its first convincing confirmation.

Bohr suggested that if an electron gained energy it would move into a higher energy orbit., farther from the nucleus. If it lost energy it would slip back into a lower energy orbit, closer to the nucleus. Bohr recognized that the idea of electrons jumping between orbits fit with the idea of light quanta in quantum theory. Perhaps an atom could absorb only photons of certain sizes.

Rutherford's Atomic Theory

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Ernest Rutherford Atomic Theory

The first exploration of the structure of the atom was made by Rutherford in 1911 making use of $\alpha$-particles. He bombarded a thin gold foil with $\alpha$-particles and found that very many $\alpha$-particles passed through the gold foil without being deflected, but some of them were deflected through large angle.

Ernest Rutherford

According to Rutherford's theory electrons move around a nucleus in an atom in certain circular orbits. But according to the classical theory of electrodynamics a moving electron will always emit energy. Thereby the electrons loses energy and ultimately falls into the nucleus. Therefore Rutherford's atom is unstable.

Neils Bohr Atomic Theory

Niels Bohr

In 1913 Neils Bohr a danish physicist provided an explanation for the occurrence of line spectra, such as those described by the Rydberg equation. Through an interesting combination of classical and quantum theory, Bohr was able to overcome the drawbacks in Rutherford's model. He suggested that the electrons in an atom revolve around the nucleus of an atom only in certain discrete orbits with no other orbits being possible. This means that an electron could occupy only certain energy levels and that it must absorb or emit energy in discrete amounts as it moves from one allowed energy level to another.

When an electron is promoted from a lower level to a higher one it absorbs a definite amount of energy. When the electron falls back to the original energy level, it emits exactly the same amount of energy it absorbed in moving from the lower to the higher energy level.

Niels Bohr Atomic Theory Discovery

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Bohr proposed that electrons would exist in orbits about the nucleus at discrete distances and each orbit would represent specific quantized energies. The energy of an electron would be proportional to its orbit.

Neils Bohr contributed a great deal to our knowledge of atomic structure by
  1. Suggesting quantized energy levels for electrons and
  2. Showing that spectral lines result from the radiation of small increments of energy (Plank's quanta) when electrons shifts from one energy level to another.

Bohr's calculations succeeded very well in correlating the experimentally observed spectral lines with electron energy levels for the hydrogen atom. However Bohr's methods of calculation did not succeed for heavier atoms.

Niels Bohr Quantum Theory

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Neils Bohr was the first to apply the quantum theory to atomic structure. The quantum theory of atomic processes was proposed by Niels Bohr in the year 1913 and was based on the atomic model proposed earlier by Rutherford.
  1. The deduction of the Balmer series for the line spectrum of hydrogen and of the Rydberg number from universal atomic constants constituted its first convincing confirmation.
  2. The development of quantum theory has only been made possible by the enormous refinement of experimental technique, which has given us an almost direct insight into atomic processes.
  3. Bohr modeled electrons as negatively charged, orbiting particles restricted to certain distances from the nucleus.
  4. However in the early 1900's a new model for the atom called the "quantum mechanical model" was developed.
  5. Although the Bohr model is useful for explaining much of the chemical behavior we encounter leaves us how electrons exist in atoms.
  6. In the quantum mechanical model orbits are replaced with orbitals. Quantum mechanical orbitals resulted from discoveries showing that the electron which was thought of as a particle also displayed properties that we normally associate with waves.
  7. The wave nature of the electron meant that its motions around the nucleus were more complex than simple circular orbits.

Niels Bohr Atomic Theory Experiment

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Bohr model a theory for the way electrons behave in atoms that explains the periodic law and links the macroscopic properties of elements to the microscopic properties of their atoms.

Bohr constructed a model of the hydrogen atom with quantized energy levels that agreed with the hydrogen emission. Bohr pictured the electron moving in circular orbits corresponding to the various allowed energy levels. He suggested that the electron could jump to a difficult orbit by absorbing or emitting a photon of light with exactly the correct energy content. Thus in the Bohr atom the energy levels in the hydrogen atom represented certain allowed circular orbits.

The Bohr model of the hydrogen atom represented the electron as restricted to certain circular orbits around the nucleus.

Bohr Model

Although the Bohr model paved the way for later theories, it is important to realize that the current theory of atomic structure is not the same as the Bohr model. Electrons do not move around the nucleus in circular orbits like planets orbiting the sun.

Niels Bohr Atomic Theory Contribution

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Niels Bohr made two huge contributions to the development of modern atomic theory.

Niels Bohr contribution to the atomic theory is listed below.
  1. First he suggested a reasonable explanation for the atomic line spectra in terms of electron energies.
  2. Second he introduced the idea of quantized electron energy levels in the atom. These levels appear in modern theory as principal energy levels they are identified by the principal quantum number n.

Bohr hypothesized that the electron in the hydrogen atom could take on a number of different orbits, at different distances from the nucleus. As long at it remained in a particular orbit the electron wouldn't gain or lose energy. But if it changed orbits a change in energy should take place.

Bohr suggested that if an electron gained energy, it would move into a higher energy orbit, farther from the nucleus. If it lost energy it would slip back into lower energy orbit, closer to the nucleus.

Niels Bohr Atomic Theory Summary

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  1. Niels Bohr suggested that electrons in an atom were restricted to specific orbits around the atoms nucleus.
  2. Bohr argued that an electron in a given orbit has a constant energy, thus he named these orbits energy level.
  3. When an electron gains energy, it can use this energy to jump from a lower energy orbit to a higher energy orbit.
  4. When an electron falls from a higher energy orbit to a lower energy orbit it releases energy in the form of light.
  5. White is not a color of light itself, but rather results when light of every other color is mixed together.
  6. In Bohr's model electrons can only exist in certain orbits and thus, can only have certain energies. As a result we say that the energies of the electron are quantized.
  7. Bohr used the formula En = $\frac{-Rhe}{n^{3}}$ to predict the energy level of an electron in the nth energy level of a hydrogen atom.
  8. Because the electron is only allowed to exist at certain  energy levels according to the Bohr model, there are only a few possible energies of light which can be released when electrons fall one energy level to another. Asa result the Bohr model explains why atomic spectra are discontinuous.
  9. The Bohr model successfully predicts the four colored lines in hydrogen's atomic spectrum, but it fails miserably when applied to any atom with more than one electron. This is due to the difference between the laws of classical physics and the laws of quantum physics.
  10. The Bohr model is no longer accepted as a valid model of the atom.