5.2. The Bohr and Quantum Mechanical Models of the Atom http://www.ck12.org
certain substances were heated, they emitted only specific wavelengths of light. In other words, the spectra were
discontinuous. In particular, the emission spectrum of hydrogen included only four wavelengths within the visible
light range, which could be seen separately by passing the emitted light through a prism (Figure5.6).
FIGURE 5.6
Hydrogen emission spectrum.Planck’s interpretation of this phenomenon was that the emissions were quantized –they were only emitted in fixed
and predictable intervals (Haendler 1982; Goodney 1991). This was described mathematically by the following
expression, E = hν, where E is the energy of a single photon, h is Planck’s constant, andνis the frequency of the
electromagnetic wave. Because of this influential work on photons and quanta, Planck was awarded the Nobel Prize
in Physics in 1918.
Bohr’s Atomic Model
Following the discoveries of hydrogen emission spectra, the Danish physicist Niels Bohr (1885–1962) proposed a
new model of the atom in 1915. Bohr suggested that electrons do not radiate energy as they travel around the nucleus
but exist in states of constant energy, which he called stationary states (Haendler 1982), orbiting at fixed distances
from the nucleus (Figure5.7). Bohr’s work was primarily based on the emission spectra of hydrogen, and it won
him the Nobel Prize in physics in 1922.
This model, also referred to as the planetary model of the atom, explained emission spectra in terms of electrons
moving between different stationary orbits that have different levels of energy. When energy is added, an electron
can jump up to a higher energy orbit, and when the electron relaxes back to a lower energy orbit, the difference in
energy is emitted as a photon of light. Using the observed frequencies of the emitted photons, the energy differences
between orbits in the hydrogen atom could be determined. Since the orbits had set differences in energy, only
certain amounts of energy could be released for any single transition. The energy released could not be any arbitrary
amount, but was instead quantized (limited to specific values). This formed the basis for what later became known
as quantum theory, which accounts for a wide range of physical phenomena that could not previously be explained.
Bohr’s work had a strong influence on our modern understanding of the inner workings of the atom. However,
although his model worked well for predicting the emissions of the hydrogen atom, it was seriously limited when
applied to other atoms. Shortly after Bohr published his planetary model of the atom, several new discoveries were
made, which resulted in, yet again, a revised view of the atom.