Atomic Models

In 1913, the Danish theorist Niels Bohr (1885–1962) developed his quantized shell model of the atom. Bohr modified Rutherford's model by hypothesizing that the electrons orbit the nucleus in specific regions of fixed size and energy. The energy of the electron depends on the size of the orbit. Electrons in the smallest orbits have the least energy. An atom is stable when its electrons occupy orbits of the lowest possible energy. The energy of an electron increases as it occupies orbits farther and farther from the nucleus.

These orbits can be thought of as the rungs of a ladder. As a person climbs up a ladder, they step on one rung or another, but not in between rungs, because a person cannot stand on air. Likewise, the electrons of an atom can occupy one orbit or another, but cannot exist in between orbits. While in an orbit, the electron has a fixed amount of energy. The electron gains or loses energy by moving to a new orbit, either further from or closer to the nucleus.

When an electron falls from the excited state to the ground state, a photon is emitted with a specific energy. The energy of the photon is equal to the energy difference between the two orbits. The energy of each photon corresponds to a particular frequency of radiation given by Planck's equation, E = hλ. Bohr was able to calculate the energy of the electron in a hydrogen atom by measuring the wavelengths of the light emitted in its line-emission spectrum. Bohr's atomic model was very stable because the electron could not lose any more energy than it had in the smallest orbit. One major problem with Bohr's model was that it could not explain the properties of atoms with more than one electron, and by the early 1920s, the search for a new atomic model had begun.