Introduction to SAT II Physics - FreeExamPapers

The Problem with Rutherford’s Model
Light and other electromagnetic waves are emitted by accelerating charged particles. In
particular, the electrons being accelerated in orbit about the nucleus of an atom release a
certain amount of energy in the form of electromagnetic radiation. If we recall the chapter
on gravity, the radius of an object in orbit is a function of its potential energy. If an
electron gives off energy, then its potential energy, and hence the radius of its orbit about
the nucleus, should decrease. But according to Rutherford’s model, any radiating electron
would give off all its potential energy in a fraction of a second, and the electron would
collide with the nucleus. The fact that most of the atoms in the universe have not yet
collapsed suggests a fundamental flaw in Rutherford’s model of electrons orbiting nuclei.
The Mystery of Atomic Spectra
Another puzzling phenomenon unexplained by Rutherford’s model, or anything else
before 1913, is the spectral lines we see when looking through a spectroscope. A
spectroscope breaks up the visible light emitted from a light source into a spectrum, so
that we can see exactly which frequencies of light are being emitted.
The puzzling thing about atomic spectra is that light seems to travel only in certain
distinct frequencies. For instance, we might expect the white light of the sun to transmit
light in an even range of all different frequencies. In fact, however, most sunlight travels
in a handful of particular frequencies, while very little or no light at all travels at many
other frequencies.
Bohr’s Hydrogen Atom
Niels Bohr drew on Rutherford’s discovery of the nucleus and Einstein’s suggestion that
energy travels only in distinct quanta to develop an atomic theory that accounts for why
electrons do not collapse into nuclei and why there are only particular frequencies for
visible light.
Bohr’s model was based on the hydrogen atom, since, with just one proton and one
electron, it makes for the simplest model. As it turns out, Bohr’s model is still mostly
accurate for the hydrogen atom, but it doesn’t account for some of the complexities of
more massive atoms.
According to Bohr, the electron of a hydrogen atom can only orbit the proton at certain
distinct radii. The closest orbital radius is called the electron’s ground state. When an
electron absorbs a certain amount of energy, it will jump to a greater orbital radius. After
a while, it will drop spontaneously back down to its ground state, or some other lesser
radius, giving off a photon as it does so.
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