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The lowest energy state an atom can be at is called its ground state. When an electron in an atom has absorbed energy it is said to be in an excited state. An excited atom is unstable and tends to rearrange itself to return to its lowest energy state. When this happens, the electrons lose some or all of the excess energy by emitting light. Light is only emitted at certain frequencies, each corresponding to a particular electronic transition within the allowed states.
The Bohr model is used to account for the spectrum of the hydrogen atom, but the basic idea is the same for all elements. The single electron in hydrogen revolves around the nucleus in one of a limited number of circular orbits. When it is in the orbit closest to the nucleus it is in its ground state, this electron is in the valence band of the element.
When hydrogen is heated, or some other energy is being applied to it, the atom absorbs the energy and the electron becomes excited and "jumps" to an orbit farther from the nucleus. In other words, it goes up to a higher energy state. The more energy is applied to the atom, the higher in states the electron goes until the point where it becomes a free electron and no longer part of the atom.
The lines in an emission spectrum occur when the electron loses energy, "falls back", from a higher energy state to a lower one emitting photons at different frequencies for different energy transitions.
You might want to take a look at the movie for a more visual explanation. You will need Shockwave installed for your browser in order for it to run. It should run on the Vectras, but it will not run on the Sun workstations. Also, for better viewing, use the "full screen" option on your browser. After you're done viewing, you will need to use the "back" button on your browser to return to this page.
Another movie you might want to look at would be the demo for a silicon atom. If you're still curious, you can try another website that goes more in detail: Review Chemistry