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Chemistry
Atoms and Atomic Structure
Niels Bohr

# What is the Bohr model of the atom?

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The Bohr model of the atom is based on four fundamental postulates. Although he did not claim to explain or prove these postulates, Bohr found that by applying them, he could predict the properties of single-electron atoms and ions to an amazing degree of accuracy. Using the Bohr model, it is possible to accurately predict the energy levels of all one-electron atoms and ions, such as H, He+, Li2+ Be3+, B4+, C5+, etc.

The four key postulates of Bohr's theory are:

1) Atoms have well-defined electron orbits.

3) Electrons travel in circular orbits with specific angular momenta, and only certain values are possible (angular momentum is quantized).

4) As electrons go from one orbit to the next, energy is either absorbed or released by the atom.

The quantized angular momentum can be written as:

I = n * (h/2*pi)

where I is the angular momentum, n is any integer, h is Planck's constant, and pi is the number pi, or 3.14159.

See Web Links and Related QuestionsSee the Related Questions and Web Links to the left for the answer. Overview of the Bohr ModelNiels Bohr proposed the Bohr Model of the Atom in 1915. Because the Bohr Model is a modification of the earlier Rutherford Model, some people call Bohr's Model the Rutherford-Bohr Model. The modern model of the atom is based on quantum mechanics. The Bohr Model contains some errors, but it is important because it describes most of the accepted features of atomic theory without all of the high-level math of the modern version. Unlike earlier models, the Bohr Model explains the Rydberg formula for the spectral emission lines of atomic hydrogen.

The Bohr Model is a planetary model in which the negatively-charged electrons orbit a small, positively-charged nucleus similar to the planets orbiting the Sun (except that the orbits are not planar). The gravitational force of the solar system is mathematically akin to the Coulomb (electrical) force between the positively-charged nucleus and the negatively-charged electrons.

Main Points of the Bohr Model
• Electrons orbit the nucleus in orbits that have a set size and energy.
• The energy of the orbit is related to its size. The lowest energy is found in the smallest orbit.
• Radiation is absorbed or emitted when an electron moves from one orbit to another. Bohr Model of HydrogenThe simplest example of the Bohr Model is for the hydrogen atom (Z = 1) or for a hydrogen-like ion (Z > 1), in which a negatively-charged electron orbits a small positively-charged nucleus. Electromagnetic energy will be absorbed or emitted if an electron moves from one orbit to another. Only certain electron orbits are permitted. The radius of the possible orbits increases as n2, where n is the principal quantum number. The 3 → 2 transition produces the first line of the Balmer series. For hydrogen (Z = 1) this produces a photon having wavelength 656 nm (red light).
Problems with the Bohr Model
• It violates the Heisenberg Uncertainty Principle because it considers electrons to have both a known radius and orbit.
• The Bohr Model provides an incorrect value for the ground state orbital angular momentum.
• It makes poor predictions regarding the spectra of larger atoms.
• It does not predict the relative intensities of spectral lines.
• The Bohr Model does not explain fine structure and hyperfine structure in spectral lines.
• It does not explain the Zeeman Effect.
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