[positiveion]

I've been watching lots of videos about atomic spectra on youtube, and they all essentially explain the same thing.

There's a ground state and once the electron is excited it goes up a few energy shells and as a result of being unstable, drops back down again (unless it is further excited).

I understand all of that, however, I don't really understand what would happen if there is more than one electron. Do they all get excited? If so do they all jump up and then fall back down? Do only the valence electrons get excited?

Thanks!

[Borek]

Any electron may get excited. It may also happen that several electrons get excited at once. Usually we observe effects of valence electrons excitation.

[Ranadeep]

The electrons excites when it absorbs particular energy of a photon colliding with it .. Photons are nothing but tiny massless corpuscles in light ..

electron(s) can excite depending on the energy of photon you've sent to it .

Generally valence shell electrons having high energy require less energy to excite than the inner core electrons which are strongly attracted to nuclear attraction which can be prove mathematically using Bohr Energy Formula 

[positiveion]

A question from my textbook which I don't know how to do:

Why do atomic spectra suggest that electrons orbit the nucleus in fixed energy levels or shells?

The only thing that half way explains it is a line that says "the spectral lines correspond to the energy given out when electrons return to the first shell. But I don't really understand how/why..