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Electrons absorb photons and jump to higher energy levels or excited states (or a higher energy orbit). They absorb only those photons which have energies equal to the energy gap between their current state and the particular orbit they are going to jump to.
How do electrons know what is the energy of the higher orbit ? And if they don't know then how is it that they absorb only certain photons ?

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  • $\begingroup$ I believe this question is answered here: physics.stackexchange.com/q/257319 $\endgroup$ – Jake Watrous Feb 8 '17 at 0:33
  • $\begingroup$ @JakeW No this is different from that. I want to ask how does the electron have a knowledge of energies of orbits in which it is not present ? Ideally it should know nothing except its present state right ? $\endgroup$ – Raghav Feb 8 '17 at 0:44
  • $\begingroup$ Electrons are not sentient. However, an electron in a quantum state is under the influence of a HamiltonIan and the HamiltonIan controls all possible states, not just the occupied ones. When such an electron absorbes a photon, the level to which it jumps is determined by the energy and polarization of the photon. The photon need not be an exact energy match to the level difference (see Raman scattring). $\endgroup$ – Lewis Miller Feb 8 '17 at 1:11
  • $\begingroup$ Don't forget that we're really talking about excitations of the atom, not the electron. The atom makes the transitions that it's allowed to, and the electron goes along for the ride. $\endgroup$ – garyp Feb 8 '17 at 2:35
  • $\begingroup$ Also I believe all the valence electrons are excited together to the new energy level. $\endgroup$ – Bill Alsept Feb 8 '17 at 6:13
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Its similar to playing a note on a guitar, does the string know what sound to make? No, the string reacts to its tension, mass an input energy. The string will play harmonics of the main note depending upon how hard it is struck. The electron is bound in the atom ( or atoms in a molecule) and can only vibrate at certain frequencies (which determine its energy).

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To start with there is no meaning of talking of an electron and a nucleus in the correctly described quantum mechanical state of an atom. The electron is in fixed ( within a width) energy orbitals , probability loci, in a reference system where the nucleus is considered at rest. If one considers the electron at rest , it is the nucleus which exists in orbitals.

It is the atom that is in the ground state or will be open to accept a photon energy (within a width) and go to the excited state.

So there is no anthropomorphic electron which "knows" about energy levels. There is an atom that can be excited to a higher state by absorbing the appropriate energy photon.

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It doesn't know!!

But the amount of energy it was given determines what stage the electron goes to.

It's been excited by the energy absorbed, forcing it to jump to the next level.

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