Guest author, Thomas Kennedy, features a twice-monthly series, Quantum Leap, wherein he guides readers through the fascinating world of quantum mechanics. This is issue 011.
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(Continued from issue 010 – Charge!)
But there appeared something terribly wrong with Rutherford’s model of the atom. The theory of electricity and magnetism predicted that opposite charges attract each other and the electrons should gradually lose energy and spiral inward. Moreover, physicists reasoned that the atoms should give off a rainbow of colors as they do so. But no experiment could verify this rainbow.
In 1912 a Danish physicist, Niels Bohr, came up with a theory that said the electrons do not spiral into the nucleus and came up with some rules for what does happen. (This began a new approach to science because, for the first time, rules had to fit the observation regardless of how they conflicted with the theories of the time.)
Bohr said, “Here’s some rules that seem impossible, but they describe the way atoms operate, so let’s pretend they’re correct and use them.” Bohr came up with two rules which agreed with the experiment:
RULE 1: Electrons can orbit only at certain allowed distances from the nucleus.
RULE 2: Atoms radiate energy when an electron jumps from a higher-energy orbit to a lower-energy orbit. Also, an atom absorbs energy when an electron gets boosted from a low-energy orbit to a high-energy orbit.*
Bohr’s atom for Hydrogen
The electron can exist in only one of the orbits. (The diagram shows only five orbits, but any number of orbits can theoretically exist.)
Light (photons) emit whenever an electron jumps from one orbit to another. The jumps seem to happen instantaneously without moving through a trajectory.
The examples above show only two possibilities from Rule 2.
By the 1920s, further experiments showed that Bohr’s model of the atom had some troubles. Bohr’s atom seemed too simple to describe the heavier elements. In fact it only worked roughly in these cases. The spectral lines did not appear correct when a strong magnetic field influenced the atoms.
*Compiled by Jim Walker, last revision 2004.
Next Issue: “Uncertainty”
Watch for Issue #12 of Thomas’ “Quantum Leap”, on April 16, 2010.
You can access all previous issues of “Quantum Leap”, here.
Tags: atom, hydrogen, Neils Bohr, Quantum Mechanics