I need some clarification in my understanding of HUP with respect to electron in an atom. This is my current understanding,

  1. Position of electron here means which orbit (Quatized orbits around nucleus) electron is located
  2. Momentum indicates the spin, how fast the the EMF rotates due to energy of an electron
  3. HUP says that if position i.e., the orbit is precisely known, then its momentum i.e, the spin is less precisely known

I believe my understanding is incorrect. The reason is it leads to a contradiction. If position is precisely known, we would know its energy level (energy level is related to orbit). If energy level is known we can figure out the momentum (because momentum and energy are directly related). Contradiction!


It is very important to differentiate between the classical orbit of an electron (like a planet around a star), and the QM orbital.

As per our currently accepted theory, the Standard Model, together with QM, the electron exists in an orbital around the nucleus at a certain energy level. AS per QM, the electron could be anywhere in this orbital at a certain energy level with different probabilities, the wavefunction describes the distribution of the position of the electron in all of space.

Now the answer to your question is why the electron is in a stable energy level (how the HUP is involved and the momentum and position):

  1. the EM charge of the electron keeps it close to the nucleus (protons)

  2. the kinetic energy of the electron keeps it away from the nucleus

  3. as the electron would get too close to the nucleus, its position would be known with more certainty, and the HUP will cause its momentum to be higher (be known with lesser certainty), so the electron gains momentum, and that keeps it away from the nucleus

Now these three forces are balancing out, and the electron is in a stable orbital at a certain energy level as per QM.

  • $\begingroup$ @Szendrei : Thanks. Uncertainity Principle talks about the range of momentum being unknown. In the case you explained, electron 'gains' momentum. Why can't it lose momentum and collide with proton. That too satisfies HUP,right? $\endgroup$ – Karthick S May 25 '19 at 18:05
  • $\begingroup$ @KarthickS correct. The classical, orbit view could not explain why it is not colliding with the proton. As per QM, the HUP explains, as it gets too close to the proton, it gains momentum (and that keeps it away from the proton). $\endgroup$ – Árpád Szendrei May 25 '19 at 18:08

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