Niels Bohr

Born on October 7, 1885, Niels Bohr won the Nobel Prize in physics in 1922 for proposing a structure for the atom and his work in quantum mechanics.

Before Bohr physicists knew that the structure of an atom consisted of a small dense nucleus orbited by electrons. Earnest Rutherford in 1911 published the results of an experiment in which alpha particles emitted by the decay of radium were used to bombard a thin piece of gold foil. The results of the experiment showed that a small amount of the alpha particles were deflected and did not penetrate the gold foil. Rutherford hypothesized that these alpha particles were deflected by the small, hard nuclei of the gold atoms. This led him to propose that atoms were composed of small nuclei surrounded by orbiting electrons that orbited the nuclei in a similar manner to the way planets orbit stars. This model is sometimes called the planetary model (wikipedia entry).

In 1913 Niels Bohr proposed a model for atomic structure where:

1. Electrons orbit nuclei only in certain orbits: orbits set at discrete distances from the nucleus.

2. Electrons can change orbitals, but in doing so they must either absorb (when moving to a higher orbital, further from the nucleus) or emit energy (when moving to a lower orbital, closer to the nucleus).

3. The frequency of the light emitted by an electron changing orbitals is related to the period of the orbital.

The emission of light by electrons falling back into lower orbitals can be seen in neon lights. A gas, sealed in a tube is electrified, causing electrons (normally staying in the lowest energy levels) to move to higher energy levels (higher orbits). When they fall back to their lower, ground state they emit light. This is how neon lights work, and each gas emits a different spectrum giving each gas a distinct color.

For his work determining the structure of atoms, Neils Bohr is our Dead Scientist of the Week for the week of October 4-10, 2009.

References:

Neils Bohr Wikipedia Entry

Bohr Model, Wikipedia Entry

Carpi, Anthony, Vision Learning, Atomic Structure II