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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. $\endgroup$ 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
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    $\begingroup$ 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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Be gentle with me, I am going to try to explain orbitals in a very simple way using many similes. I do this with primary school kids so many of the words I will use aren't literally accurate but I hope the overall picture is as accurate as possible (without knowing quantum math). Please fell free to make suggestions where the overall picture deviates from "practical" reality. Please, don't bother to comment about the semantics of individually problematic words.

So:

The first thing is, don't think of electrons as particles. That may work well for certain models or math with specific purposes, but in general it actually makes them harder to understand. Second, electrons don't orbit in orbitals. That's just an unfortunate name leftover from old theories. Finally, electrons will basically be as "lazy" as they are allowed to be. They will endeavor to come to rest and "spread" their "charge" in a position and geometry around the nucleus that is as close to the nucleus as possible, but as far from any other electrons as possible. With one exception:

If given the chance, an electron will team up with another electron in their own orbital to hold the "space" with half the individual "effort". They can pull this one-time trick off by adopting complimentary wave functions thereby staying out of each other's way. This lets each of the two electrons shed some energy individually, but only two can team up this way in any given orbital space.

If there is just one electron in an atom, it is easy for it to come to rest: it "spreads" into an equidistant spherical layer (or crust) around the nucleus. It won't get any closer because its "charge" can't be "compressed" into any smaller of a space without more of a "push", which requires more energy, not less. As more electrons are added (or rather, attracted by a bigger nucleus with more "pull"), they all jockey to find that sweet-spot geometry to spread themselves into where they are as close to the nucleus as possible while being as far from each other as possible. This creates more and more complex geometries as more electrons are added.

These orbitals don't really "exist" when they aren't filled, they are simply the predictable places electrons can possibly come to rest in this rule-based system, depending on the circumstances. Sort of like Lego bricks: You can only attach them to each other in places where the studs and holes line up, there is no in-between position.

So to get to your specific question, if you add energy to the lone electron in the 2s orbital in lithium, it will no longer be able to "fit" itself in that space, and it will jump up to another, "bigger" orbital that can contain its full "charge". It isn't that this new orbital was already there, it was simply the predictable next-best resting place. But it won't stay there for long. Electrons constantly seek to find the lowest energy place to hang. As soon as you stop feeding it energy it will choose to shed its excess energy and get as close to the nucleus as possible again, falling back down to its original resting place. Interestingly, that energy will come out as light. This is how neon lights and the aurora borealis work.

Further:

This is also why certain atoms create certain molecules so well: if one electron is trying to fill up its orbital all by itself, it will gladly team up with another electron that will help it do the job with less energy. By overlapping two orbitals from different atoms, both containing electrons going it alone, each electron can actually drain off a little energy and be more lazy. It's like if two balloons found a way to overlap and share a little of their air: by overlapping they could drain off the volume of air equivalent to the volume of the overlap because it would be redundant. BUT, pretend those balloons have to be full, no matter what. So if you want to pull them apart again you must add that air back so they can both be full on their own again. That is why you need to add energy to break molecules apart. Those shared electrons need to step up and start holding their orbitals with no outside help again.

One of the coolest things is, bringing in another atom with its own electrons completely disturbs the geometries of resting places of all the outermost electrons in the first atom, so they have to re-jockey to find a new "equilibrium" to accommodate the "disturbance". This creates totally new geometries. These are called hybridized orbitals.

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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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