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I read in a textbook that after an electron jumps to a higher energy level when supplied with high temperatures or current, they again fall back to their respective energy level by losing energy and in doing so, release photons.

First question, how does this happen with details?

I had this thought: Consider the lithium atom whose outermost subshell is the 2s. Thus atom is supplied with hundreds of degrees of temperature. The valence electron is bound to get excited. It's suppoed to jump to a higher energy level. How can it when lithium only has 2 energy levels? Furthermore, do all possible orbitals preexist in every atom in the universe? Like does lithium also have the 4p subshell but it's simply empty?

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    $\begingroup$ Lithium (just like any other atom) has infinitely many orbitals and energy levels, most of them empty. Whether they exist when empty is a philosophical question. On the other hand, in what sense do they exist even when filled? An orbital is but a mathematical abstraction, after all. $\endgroup$ – Ivan Neretin Sep 5 '16 at 12:29
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    $\begingroup$ Who said lithium only has 2 energy levels? $\endgroup$ – DHMO Sep 5 '16 at 12:30
  • $\begingroup$ Have a look at the wikipedia or Hyperphysics web pages on the hydrogen atom spectrum. These give the basic ideas about energy levels which can then be extended to hydrogenic atoms (with one outer electron) such as lithium. $\endgroup$ – porphyrin Sep 5 '16 at 17:08
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there are an infinite number of atomic orbitals for each atom. Because most of them are unoccupied, they are usually not considered. When electrons gain energy, they are able to move to an orbital with a higher energy by absorbing energy. Scientists use results called emission spectra and absorption spectra in order to understand from which energy shell to which shell the electron jumping has occurred.

You also asked about preexistence. Well yes they technically do preexist. Because the results of these experiments involving the spectra mentioned above point to the fact that there are only specific energy promotions. And this can be used to calculate the energy of the orbitals. This is only for an isolated atom.

If you put many atoms together, they form what is known as an energy band. Here, you have many many more energy jumps, so they are just grouped together into a band.

Usually heating isn't the method used for such. Heating is used for ionisation energy. This is energy required to remove electrons completely from the atom.

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