In 1913, Niels Bohr came up with a new model to describe why elements gave off certain colors when they were energized by heat or electricity. He proposed a new atomic model (called the planetary model) that organized electrons into defined orbits, similar to how planets orbit the sun. His evidence came from the emission spectrum of hydrogen.

First of all, take a look at the electromagnetic spectrum.
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White light (like sunlight) will emit all colors of the rainbow that are detectable by the human eye. But when elements emit colors, these colors appear as bright lines in the spectrum. They give off light of very specific wavelengths.

Here is an interactive electromagnetic spectrum.

Follow this link to see a periodic table of emission spectra.

For more on absorption of electromagnetic radiation: see ozone and greenhouse gases.

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Bohr came up with a theory based on absorption and emission of electromagnetic energy to describe this phenomenon:
  1. An element's electrons are normally in their ground state (lowest possible energy); as they are exposed to energy (heat, electric fields, etc.) they absorb energy and jump to higher orbits about the nucleus, called excited states.
  2. Electrons do not remain in their excited states - they end up cascading down through the orbits until they reach the ground state again. While falling down to lower orbits, they release energy (called emission) in the form of photons - electromagnetic radiation - that have specific wavelengths.

This diagram is a simple model of the Bohr hydrogen atom, where the electron (green dot) was in an excited state (n = 3) and fell to a lower energy level (n = 2), releasing energy in the form of light (represented as hv).
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The Bohr model

An element's emission spectrum is so unique that it can be used to identify that element in what is called a flame test.

See this animation that demonstrates the emission theory for the hydrogen atom.




Give this Hydrogen Atom Simulator a try.