Bohr Model of the Atom and Difficulties
sometimes called the Rutherford-Bohr Model.
OVERVIEW OF
THE BOHR MODEL
Niels Bohr
proposed the Bohr Model of the Atom in 1915. Because the Bohr Model is a
modification of the earlier Rutherford Model, some people call Bohr's Model the
Rutherford-Bohr Model.
The modern
model of the atom is based on quantum mechanics. The Bohr Model contains some
errors, but it is important because it describes most of the accepted features
of atomic theory without all of the high-level math of the modern version.
Unlike earlier models, the Bohr Model explains the Rydberg formula for the
spectral emission lines of atomic hydrogen.
The Bohr Model
is a planetary model in which the negatively-charged electrons orbit a small,
positively-charged nucleus similar to the planets orbiting the Sun (except that
the orbits are not planar). The gravitational force of the solar system is
mathematically akin to the Coulomb (electrical) force between the
positively-charged nucleus and the negatively-charged electrons.
MAIN POINTS OF
THE BOHR MODEL
Electrons travel round the nucleus in orbits that have a set size and energy.
The energy of
the orbit is related to its size. The lowest energy is found in the smallest
orbit.
Radiation is
absorbed or emitted when an electron moves from one orbit to another.
BOHR MODEL OF
HYDROGEN
The simplest
example of the Bohr Model is for the hydrogen atom (Z = 1) or for a
hydrogen-like ion (Z > 1), in which a negatively-charged electron orbits a
small positively-charged nucleus. Electromagnetic energy will be absorbed or
emitted if an electron moves from one orbit to another.
Only certain
electron orbits are permitted. The radius of the possible orbits increases as
n2, where n is the principal quantum number. The 3 → 2 transition produces the
first line of the Balmer series. For hydrogen (Z = 1) this produces a photon
having wavelength 656 nm (red light).
DIFFICULTIES WITH
THE BOHR MODEL
It violates
the Heisenberg Uncertainty Principle because it considers electrons to have
both a known radius and orbit.
The Bohr Model
provides an incorrect value for the ground state orbital angular momentum.
It makes poor
predictions regarding the spectra of larger atoms.
It does not
predict the relative intensities of spectral lines.
The Bohr Model
does not explain fine structure and hyperfine structure in spectral lines.
It does not
explain the Zeeman Effect.
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