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Can any one define quantum jump 9 quantities jump of an electron ? I know it it is a silly question but can anyone please explain me in detail.I am a learning about the structure of atom and I want some help. Can anyone help.

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up vote 4 down vote accepted

Electron's quantum jump is the same thing as an atomic electron transition.

At the beginning, the electron has one energy and sits at some level which may be represented by some "typical distance" from the nucleus.

At the end, it has another value of the energy, so a different "typical distance from the nucleus".

Quantum mechanics, the theory of atoms etc., implies that not all values of energy but only some particular values are allowed. This is why the electron can't gradually slowly walk or creep from one energy (or one orbit) to another. It has to "jump" i.e. omit all the values in between. It directly changes from one level of energy to another, i.e. by absurbing a photon to obey the energy conservation law.

Energy jumps are behind the emission and absorption of light by atoms.

To learn everything about the jumps and why the energy levels are discrete etc., one needs to learn quantum mechanics.

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If you weren't Luboš Motl I'd say that this answer is wrong, but I'll at least say that I don't understand what you mean by it. Unitary evolution is continuous, and that includes energy level transitions. The "quantum jump" is a concept from the Bohr model, pre-QM, and has been obsolete since the 1920s. There's no rule in QM that a particle can't be in a superposition of eigenstates of different energies, and the excited states are not energy eigenstates anyway once you allow the possibility of emitting a photon. – benrg Aug 3 '14 at 16:33
Dear @benrg, be sure that the term "quantum jump" is being used even in the new full quantum mechanics, see the first paragraph at - not just in Bohr's theory. Concerning Bohr's theory, it was a toy model for quantum mechanics and some things were supposed to be analogous to what they are in the full QM theory but it never quite worked. ... Otherwise in QM, unitary evolution is mathematically continuous but the actual observables such as energy often evolve discontinuously. They have to 'cause levels in between discrete ones don't exist. – Luboš Motl Aug 3 '14 at 17:08
@benrg The existence of quantum jumps is an experimental fact. This standard review on the topic has a more detailed explanation, and a list of references. The existence of discontinuous jumps is intimately connected with the discontinuous measurement process, therefore your intuition about continuous unitary evolution is not quite appropriate. – Mark Mitchison Aug 3 '14 at 22:24
No, it is not silly. The situations are indeed absolutely analogous, like any other situation in QM. There is no intermediate value of $j_z$ between $-\hbar/2$ and $+\hbar/2$. There are pure states where the spin is sharply polarized with respect to another axis different from $+z$ or $-z$ but these states don't have a well-defined $j_z$, they are not eigenstates. If you ask what is their $j_z$, it is either $+1/2$ or $-1/2$ with some odds but nothing in between. – Luboš Motl Aug 4 '14 at 7:08
@benrg I thought the discussion was about whether the energy changes continuously, not the wave function. In my opinion, as soon as you ask how an observable quantity changes, you are implicitly asking a question about measurement. How else could one talk about observables? The question about how the energy of an individual atomic system evolves can only be answered by continuously measuring the atom in its energy eigenbasis (approximately), in which case the quantum jumps are seen. One could choose any other basis, but then one doesn't learn about the energy, but some other observable(s). – Mark Mitchison Aug 4 '14 at 10:56

(add my comment as an answer)

It all depends on the time-energy uncertainty relation $$ (\Delta t) (\Delta E) \ge ℏ/2 $$
(see for example here and here).

Classicaly a particle can access only system (energy) states which are compatible with its (current) energy. Actually this is still true in quantum mechanics, the difference is the time-energy uncertainty which provides a twist if you like.

A particle can transition to another (energy) state by a change of its energy. Due to the uncertainty relation, given a (small) time-frame, some energy can be given so to say, to the particle, so the jump to another state (which was classicaly unlikely) is now possible and can happen. Of course this works both ways.

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