The problem with finding out that electrons where capable
of existing
only at certain energy levels was coming up with a model to explain
these
levels. In 1913 Neils Bohr (18851962), a Danish physicist,
proposed a theoretical model for the hydrogen atom. He chose hydrogen
because
its atoms are the simplest, having only one electron about the nucleus,
and because it produces the simplest spectrum with the fewest
lines.
In his model, Bohr imagined the electron to move around the nucleus
following
fixed paths, or orbits, much as a planet moves around the sun. His
model
also restricted the sizes of the orbits and the energy that the
electron
could have in a given orbit. The equation Bohr derived for the
energy
of the electron included a number of physical constants such as the
mass
of the electron, its charge, and Planck's constant. It also contained
an
integer, n, that Bohr called a quantum number. Each of
the
orbits could be specified by its value of n. 
Bohr's Atomic Model 
Bohr found that the electron had the least energy when n
=
1. which corresponds to the first Bohr orbit. This lowest energy state
is called the ground state. This orbit also brings the electron closest
to the nucleus. 

When the hydrogen atom absorbed energy, as it does in a
gas discharge
tube, the electron is raised from the orbit n = 1 to a higher
orbit
such as n = 2 or n = 3 or even higher. Then when
the
electron drops back to a lower orbit, energy is emitted in the form of
light. Since the energy of the electron in a given orbit is fixed, a
drop
from one particular orbit to another, say from n=2 to n=1, always
releases
the same amount of energy, and the frequency of light emitted because
of
this change in energy is always precisely the same.


Bohr's model of the atom was both a success and a
failure.
It successfully predicted the frequencies of the lines in the hydrogen
spectrum, so it seemed to be valid. Nevertheless the model was a
total failure when it tried to predict energy levels for atoms with
more
than one electron. Still the theory held some validity and is
still
used to introduce students to the concept of orbital shells and the
first
quantum number "n". 