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Stylised Lithium Atom by Halfdan
One flaw in Rutherford’s atom was that it couldn’t explain the presence of spectral lines as observed in an electrical gas discharge.
To elaborate, it is known that accelerated charges release electromagnetic waves. And since electromagnetic waves carry energy (recall E = h f), accelerated charges also release energy.
Now, Rutherford’s atom, which has electrons orbiting the nucleus, definitely has accelerated charges (i.e., the orbiting electrons). In other words, Rutherford’s atoms should be releasing electromagnetic waves and energy. If that were so, then the electrons, which would be losing energy, would consequently have to spiral to the center of the atom. Eventually, you’d have collapsing atoms everywhere.
It was evident that Rutherford’s atom wasn’t stable enough to truly exist.
What about the spectral lines mentioned earlier? Yes, I almost forgot that. If Rutherford atoms did release energies the way they were supposed to, then we’d be observing a continuous spectrum instead of spectral lines.
In the Bohr atom, Bohr suggested that the electrons just stayed in specific orbits and would only jump to an orbit of lower energy when the atom emitted electromagnetic waves (energy). Also, each atom could only support specific orbits (or energy levels). Thus, this configuration would allow only for specific electromagnetic wave emissions for a particular atom.
That is, the allowed released energies and electromagnetic wave frequencies would only take discrete values as opposed to the more continuous one in Rutherford’s model. This explained the emission of spectral lines. The same principle is used to explain the presence of absorption lines.
These are observed when an atom absorbs certain energies and the electrons jump up to an orbit with higher energies. When viewed through a spectrometer, what used to be the location of colored spectral lines are inhabited by black lines.
The Bohr atom, which exhibited discrete or quantized values, was one of the predecessors of quantum mechanics. However, since it borrows a lot of its structure from Rutherford’s, it is sometimes known as the Rutherford-Bohr atom.
While the currently accepted model of the atom has a cloud of electrons surrounding the positively charged nucleus, the Rutherford-Bohr atom still allows for a simplified way to explain certain characteristics of the atom.
Universe Today has a nice collection of Atomic Model articles. If you want an article that supports the more recent atomic model, there’s one that talks about the electron cloud model.
NASA also has some related articles:
Expansion of the Bohr Model
Atoms and Light Energy
Here are some interesting episodes from Astronomy Cast:
Quantum Mechanics
Energy Levels and Spectra
Sources:
UT-Knoxville
Wikipedia
University of Oregon
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