http://www.ck12.org Chapter 5. Electrons in Atoms
to absorb any amount of energy, and it would not be limited to discrete wavelengths. Likewise, when the atoms
relaxed back to a lower energy state, any amount of energy could be released. This would result in what is known
a continuous spectrum, where all wavelengths and frequencies are represented. White light viewed through a prism
and a rainbow are examples of continuous spectra. Atomic emission spectra provided further evidence for the
quantized nature of light and led to a new model of the atom based on quantum theory.
Work with simulated discharge lamps at http://phet.colorado.edu/en/simulation/discharge-lamps.
The following video shows a lab demonstration of the atomic emission spectra: http://www.youtube.com/watch?v=9
55snB6HLB4 (1:39).
MEDIA
Click image to the left for more content.Bohr Model of the Atom
The figure below (Figure5.9) helps explain the process that occurs when an atom is excited and relaxes back to a
lower energy level.
FIGURE 5.9
As an excited atom with energy equal toE 2 falls back down to energyE 1 ,
it releases energy in the form of a photon of electromagnetic radiation.
The energy of the photon isEphoton=E 2 – E 1 =hν.Two energy states of an atom are indicated above (Figure5.9): a lower energy state (E 1 ) and a higher energy state
(E 2 ). As the atom moves from theE 2 excited state down to the lower energyE 1 state, it loses energy by emitting a
photon of radiation. The energy of that photon(Ephoton=hν)is equal to the gap in energy between the two states,
E 2 −E 1. The observation that the atomic emission spectrum of hydrogen consists of only specific frequencies of
light indicates that the possible energy states of the hydrogen atom are fixed. This suggested that the electrons in a
hydrogen atom were arranged into specific energy levels.
In 1913, Danish physicist Neils Bohr (1885-1962) proposed a model of the atom that explained the hydrogen atomic
emission spectrum. According to the Bohr model, which is often referred to as a planetary model, the electrons
encircle the nucleus of the atom in specific allowable paths called orbits. When an electron is in one of these orbits,
its energy is fixed. The ground state of the hydrogen atom, where its energy is lowest, occurs when its single electron
is in the orbit that is closest to the nucleus. The orbits that are farther from the nucleus are all higher energy states.
The electron is not allowed to occupy any of the spaces in between the orbits. An everyday analogy to the Bohr
model is the rungs of a ladder. As you move up or down a ladder, you can only occupy specific rungs and cannot be
in the spaces between the rungs. Moving up the ladder increases your potential energy, and moving down the ladder
decreases your energy.