Your final goal is to stimulate the chromium atom to emit another photon, which it will do when the electron drops from energy E 2 to E 1. You do
this by firing a new photon at the chromium. Which color do you think will cause the emission of an additional photon?
If you see two photons moving to the right and the electron in the chromium atom returns to energy level E 1 , you have successfully simulated
the workings of a laser. Congratulations! The second photon you fired by pressing GO has been “amplified” since it results in two photons
moving through the ruby. It has taken advantage of the energy stored in the atom to do this.
As you conduct your experiments, you may note that the details of the laser process differ from material to material. With neon, the crucial
transition is from the highest metastable level to a middle level. With chromium, the material used for this transition, the crucial transition is from
a middle metastable level back to the lowest energy state.
36.8 - Bohr atom
Bohr atom: The atom consists of a nucleus
surrounded by electrons orbiting it at specific
radii and energy levels.
Einstein showed that light was quantized. The Danish physicist Niels Bohr proposed a
model of the atom in which the energy of electrons was quantized, and could only exist
at certain values. Together, these theories explained the frequencies of spectral
emission and absorption lines.
Before discussing Bohr’s theory, we will briefly explain some work that preceded his,
and then explain his crucial hypotheses.
In 1897, the scientist J. J. Thomson showed that the “rays” often observed flowing
between charged electrodes in a vacuum were streams of negatively charged particles.
In other words, he discovered the electron. His discovery caused scientists to update
their model of the atom. Some theorized that atoms consisted of a “mix” of negative
particles and positive regions, like negatively charged chocolate chips embedded in
positively charged cookie dough. (In fact, Thomson called it the plum pudding model,
after the plums scattered throughout a pudding.)
However, in 1910 Ernest Rutherford conducted experiments that led scientists to reject
the plum pudding model. He fired alpha particles at a thin gold foil, expecting them to
sail through. (An alpha particle consists of two protons and two neutrons, and is
positively charged.)
Rutherford discovered that although most of the particles passed through the foil with
minimal deflection, a few had violent collisions. These particles were deflected at
extreme angles, or even rebounded straight back.
Only a dense, positive nucleus could explain this result. Rutherford’s subsequent
analysis led him to conclude that this tightly packed nucleus must be surrounded by
electrons that were orbiting the nucleus at relatively great distances, a model eerily
similar to the solar system. In short, he developed the basis of a model that is still
commonly used today to describe the atom.
During the period of these discoveries, the physicist Robert Millikan measured the
magnitude of the elementary charge, which is the amount of charge of an electron or a
proton. In a little over a decade, the basic model of a positively charged nucleus orbited
by negatively charged electrons had been established, as had the charge of an
electron.
The model of the atom was radically advanced by the work of Thomson, Rutherford and
Millikan. However, physicists remained puzzled by a paradox stemming from their
understanding of electromagnetic theory and orbital mechanics.
Electromagnetic theory predicts that accelerating charges emit electromagnetic
radiation. Electrons circling around the nucleus of an atom are constantly accelerating
because they are constantly changing direction; this is similar to how the electrons in an
antenna repeatedly accelerate back and forth as they oscillate over its length in simple
harmonic motion. If orbital electrons were emitting radiation due to their acceleration,
they would be losing energy, and they should eventually crash into the nucleus. The
analogous effect is witnessed with a satellite orbiting the Earth: If it continually loses
energy due to atmospheric resistance, its orbital radius decreases, and it eventually
crashes.
However, since most atoms are stable (phew!), electrons are not “crashing” into nuclei,
but rather are maintaining orbits of a constant radius. Bohr could not explain why the
electrons acted as they did, but he formulated a theory that was consistent with what physicists were observing. He stated that electrons in
atoms could only exist in certain orbits called stationary orbits or stationary states. Bohr postulated that in these states the size and energy of
Bohr atom
Electron moves in circular orbit around
nucleus
Change in energy alters orbital size
Orbits are quantized