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Back in school, I've must have been 14 years old or something. Our teacher showed that different substances emits different coloured light. From the top of my head I remember copper - green and sodium - yellow. He showed more, but these will do for now.

Sodium has a valence electron in 3s. Lets say I want to check whether it is the jump from 3s to 3p and back that creates photons of this yellow wave length. Now I get puzzled. from my understandings the jump between energy levels are the source of the light.

[itex]\displaystyle E_n = -\frac{13.6}{n^2}[/itex]

But this doesn't explain why the 3p orbital is at lower energy than 3s. Since [itex]\displaystyle -\frac{13.6}{3^2} = -\frac{13.6}{3^2} [/itex]

I don't know whether I should be embarrassed by missing the obvious or flinch back because it's so complicated. How do I calculate the frequency of a jump from 3p to 3s..?

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# Determine frequency of emitted photon

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