THE BOHR MODEL OF THE ATOM
In the years immediately following Rutherford’s announcement of his nuclear model
of the atom, a young physicist, Niels Bohr, added an important piece to the atomic
puzzle.
For fifty years it had been known that atoms in a gas discharge tube emitted and
absorbed light only at specific wavelengths. The light from a glowing gas, passed
through a prism to disperse the beam into its component wavelengths, produced
patterns of sharp lines called atomic spectra. The visible wavelengths that
appeared in the emission spectrum of hydrogen had been summarized by a simple
formula. But why do atoms emit (or absorb) radiation only at certain discrete
wavelengths?
Bohr’s model of the atom explains this. Using the simplest atom, hydrogen (which
has one electron), Bohr postulated that the electron orbits the nucleus only at certain
discrete radii. When the electron is in one of these special orbits, it does not lose
energy (as the classical theory would predict). However, if the electron absorbs a
certain amount of energy, it is excited to a higher orbit, one with a greater radius.
After spending a short time in this excited state, it returns to a lower orbit, emitting
a photon in the process. Since each allowed orbit—or energy level—has a
specific radius (and corresponding specific energy), the photons emitted in each
jump also have specific energies and wavelengths. We say that the electron’s
energy levels are quantized.
The Chemistry of the
SAT Physics Test
Modern physics begins to
explore the atom, which
bridges slightly into chemistry.
That said, this is still
the SAT Physics test, and
you’ll only need the most
basic understanding of
chemistry and an awareness
of how to apply
the formulas.The energy levels within the hydrogen atom are given by the formula