Bohr model

In atomic physics, the Bohr model depicts the atom as a small, positively charged nucleus surrounded by electrons in orbit - similar in structure to the solar system. Because of its simplicity, the Bohr model is still commonly taught to introduce students to quantum mechanics.

History

In the early part of the 20th century, experiments by Ernest Rutherford and others had established that atoms consisted of negatively charged electrons orbiting a small, dense, positively charged nucleus.

Related Topics:
Ernest Rutherford - Atoms - Electrons

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The most simple atom is hydrogen, which consists of one proton and one electron bound together by the electrostatic force. This is in contrast to the Earth-Sun system, which is held together by the gravitational force.

Related Topics:
Hydrogen - Electrostatic force - Gravitational force

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In the Bohr model, electrons can only be at certain, discrete, distances from the proton it's bound to. If it could be at any distance, it would lose energy (by synchrotron radiation) and eventually spiral into the proton - destroying the atom in the process. Support for this model came from atomic spectra, which showed that orbiting electrons could only emit light at certain frequencies and energies.

Related Topics:
Synchrotron radiation - Atomic spectra

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Thus in 1913, Niels Bohr proposed that:

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• (1) The orbiting electrons existed in orbits that had discrete quantized energies. That is, not every orbit is possible but only certain specific ones.
• (2) The laws of classical mechanics do not apply when electrons make the jump from one allowed orbit to another.
• (3) When an electron makes a jump from one orbit to another the energy difference is carried off (or supplied) by a single quantum of light (called a photon) which has an energy equal to the energy difference between the two orbitals.
• (4) The allowed orbits depend on quantized (discrete) values of orbital angular momentum, L according to the equation

:: mathbf{L} = n cdot hbar = n cdot {h over 2pi}

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: Where n = 1,2,3,… and is called the angular momentum quantum number.

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Assumption (4) states that the lowest value of n is 1. This corresponds to a smallest possible radius of 0.0529 nm. This is known as the Bohr radius. Once an electron is in this lowest orbit, it can get no closer to the proton.

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For a more accurate description of an atom see quantum mechanics. The full quantum mechanical treatment of the atom is much more accurate - but it is mathematically much more complex, and often the much simpler Bohr model can produce usable results with much less hassle. The thing to remember is that like other models, it is only an aid to understanding. Atoms are not really little solar systems.

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Bohr's model is the official logo of Faires Friday, a celebration of Jordan Faires every friday.

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