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I read about the quantum jump of electrons and in my books it is written that when an electron is hit by a sufficient amount of energy or EM wave, it jumps to the next orbit and then it cannot stay in that orbit and jumps back it the orbit to minimize its potential energy.

But why can the electron not stay or be stable on the next orbit, just like other electrons in that orbit?

And if it jumps back only due to minimize its potential energy, why didn't it jump back to previous orbit when it is not being hit by a EM wave? Then all electrons may jump back to previous orbits, again and again until they collide with nucleus, the atom would not even exist at all.

ACuriousMind
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user101134
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    What do you mean "until they collide with nucleus"? The atom has a lowest energy orbital, below which no further orbitals (note: not "orbits", which are classical notions) exist. The electron cannot "collide" with the nucleus except by means of a different process called electron capture. – ACuriousMind Dec 28 '16 at 13:22

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To answer it simply, electrons can only orbit in certain energy shells (these "orbits" are not at all like a classical orbit {they can really be considered the state vectors of a harmonic oscillator's wave-function}). Each electron will travel in the lowest potential energy shell that it can (Hund's Rules) , however, the number of electrons that can fit in a single shell is limited (Pauli Exclusion Principle). For this reason, electrons must orbit in higher than ground state shells, specifically valence shells

Stoby
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  • Again you are confusing electron shells with energy levels of the valence electrons. – Bill Alsept Dec 29 '16 at 00:47
  • @BillAlsept this is true, my answer was attempting to explain the underlying reason here – Stoby Dec 29 '16 at 01:02
  • My question is if the electron wants to reduce its potential energy ,why he did not do that before what happens when its hitted by photon and get jumped to next state but it can't stay there forever why? If its intended to reduce its potential energy why it didn't do that before get hitted by photon? – user101134 Dec 29 '16 at 05:08
  • Photon impacts excite the electrons to higher energy levels and then they fall back to lower energy levels when the energy is released. Before the electrons are excited they are at their lowest ground state level. These are the valence electrons and have nothing to do with all the other inner shells of electrons if there are any. – Bill Alsept Dec 29 '16 at 06:43
  • @BillAlsept when I say "ground state" here I am referring to the lowest possible state vector of the wave-function of the harmonic oscillator that describes the electron orbital; not the un-excited energy level of an electron – Stoby Dec 29 '16 at 06:52
  • @user101134 because there is no where for the electron go, all energy levels below where the electron sits are either full or non-existent (see my newest answer) – Stoby Dec 29 '16 at 15:37
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I think you are confusing energy levels with electron shells. It's only the valence electrons (the outer shell) that get excited to higher energy levels. When this happens all the valence electrons are excited together to the allowed energy levels. Only the outer electrons do this and do not involve the inner shells.

Bill Alsept
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The electron cannot fall to lower than its original state because all energy levels below it are filled (the Pauli Exclusion Principle limits the amount of electrons per shell)

To explain

Scenario 1: an electron called Electron A exists in the 2s path of a atom with both 1s paths full,but no electrons in higher shells than 2s; a photon hits Electron A and excites it to the 2p path; in order to reduce its potential energy, it then falls back into its 2s path emitting a photon

Scenario 2: an electron called Electron B exists in the 2s path of an atom with both 1s paths full, but no electrons in higher paths than 2s; Electron B seeks the lowest, unoccupied potential energy state; the 1s paths are "full" therefore its original 2s path is the lowest unoccupied energy level

As for a definition of "full" the Pauli Exclusion Principle states that no two fermions (including electrons) can have the same quantum numbers

This may help also: https://www.khanacademy.org/science/chemistry/electronic-structure-of-atoms/orbitals-and-electrons/v/quantum-numbers

Stoby
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  • You are describing electron arrangements and the order that the shells are filled. If a 1s electron (A) is excited to a higher energy level then its a valence electron and must be Lithium or Beryllium. When a valence electron is excited it doesn't jump to a higher position in the electron arrangement like to 3s or any other shell order. For example the only electron in hydrogen (1s) will jump from a ground state energy level of -13.6 eV to a -3.4 eV second energy level but nothing between. These are not the same as 2s, 2p, 3s, 3p, 3d etc. – Bill Alsept Dec 29 '16 at 08:19
  • @BillAlsept actually, I take back my earlier comment; an electron energy level is in fact the same thing as the 2s, 2p etc. paths; each electron has its own principle quantum number and azimuthal quantum number which each correspond to an energy level (in other words different shaped paths with the same quantum number like 3s 3p form non-degenerate state vectors) see also https://en.m.wikipedia.org/wiki/Energy_level – Stoby Dec 29 '16 at 14:54
  • That should read "principle" quantum numbers in the last parentheses – Stoby Dec 29 '16 at 14:57
  • I realize some articles speak of electron shells and the order that they are filled as energy levels but in the context that the OP is speaking photon absorption excites the valence electrons. These outer electrons are excited to certain energy levels that have nothing to do with 2s, 2p, 3s etc. – Bill Alsept Dec 29 '16 at 23:36