I understand the principle of how light is emitted from an atom. What I don't know is why neon atom is red and copper is green when exited? Is is the distance between the electron to protons or the number of electrons? When halogen bulbs get old the gas ionizes and emits a purplish hue compared to its normal white. Would a neon atom emit a different color if ionized?

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  • $\begingroup$ The color of light emitted is dependent on the difference in energy leve!s of the electron before and after it "jumps" and emits the photon. So it varies from atom to atom. $\endgroup$ – user108787 Apr 29 '16 at 23:35

The color of the photon is related to its frequency $f$, which can be related to the energy of the photon by the expression $E = hf$, where $h$ is Planck's constant. Thus the different colors of the emitted photons describes their different energies.

The next step is to determine why specific elements emit certain colors. This has to do with the different energy levels of the electrons "orbiting" the nucleus. When an electron drops from a high energy orbital to a low energy orbital the difference in energy results in an emitted photon. There are many rules regarding the allowed transitions, hence the allowed energy of photons that are emitted, and these vary by element. What you are observing are the allowed transitions of each specific element.

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  • $\begingroup$ Sorry about the cross posting. $\endgroup$ – user108787 Apr 29 '16 at 23:37
  • $\begingroup$ "The color of the photon is related to its frequency $f$", is reasonable true, but only because you restrict it to light of a single frequency by considering only one photon. Light including contributions of multiple frequencies does not follow so simply a understanding because "color" is a sensory result rather than a physical property of the light and the processing your brain does on visual input is considerable. $\endgroup$ – dmckee --- ex-moderator kitten Apr 30 '16 at 0:08
  • $\begingroup$ It is true that we would need to consider the entire emission spectrum, however for simplicity it is assumed that the observed color is due to the dominance in emission of photons a certain wavelength (due to the corresponding electron transitions). Pure elements will only have a few strong emission lines and the strongest of these will usually dominate the color. Compounds made of multiple atoms will have emission from many elements and then the mixing of emission lines (and the corresponding sensory response) will be a dominant factor. $\endgroup$ – Snyder005 Apr 30 '16 at 1:58

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