# How to think about speed or velocity of an electron (in an atom)?

One answer to this question explains that "velocity of electrons has no meaning" while another says that "it can be argued that they don't move around atoms at all". And then in another post it is answered that electrons in atoms do indeed have a speed associated with their kinetic energy. Granted, speed is not velocity, but is the expectation value $$\langle\hat{p}^2/m^2\rangle$$ not what one can think of as akin to a classical "speed" (squared). Is a measured value of $$\hat{p}/m$$ not akin to a classical velocity* (or can this not be measured, not even a probability distribution)?

How is (a chemist) to think of angular momentum of an electron in an atom (specifically orbital angular momentum)? How completely should one abandon the classical idea of velocity as part of momentum when discussing quantum particles? This post has an answer that provides a geometric argument about "transforms under rotations". Isn't there a more (classically) appealing picture?

$$*$$ ignoring relativity

• No, there is no classically appealing picture. Bohr’s model of atoms as little solar systems isn’t a good model. – G. Smith Dec 16 '19 at 0:01
• If you can briefly explain what role orbital angular momentum plays in chemistry, we might be able to suggest a way to think about it. – G. Smith Dec 16 '19 at 0:08
• There are two questions. I'll focus on the one in the title. Although QM does not give orbits, it does give the total, of all electrons, velocity distribution as $\psi^* {\bf p} \psi$ + cc. – my2cts Dec 16 '19 at 8:36
• @G.Smith To give a general example, since it is a conserved property it plays a role in establishing transition rules (chemists are taught to varying extents about these principles, including about angular momentum coupling): en.wikipedia.org/wiki/Selection_rule#Angular_momentum . I wish I could give a more concrete example, but this question is just attempting to get a clearer and self-consistent description of what velocity might mean in the context of electrons in atoms. – Buck Thorn Dec 16 '19 at 9:49
• Let's say you have two distinct electrons (in different but otherwise identical atoms to avoid complications). One has angular momentum zero. The other has angular momentum (as a quantum number) of 1. Certainly there must be some sense in thinking of one as "moving more rapidly" than the other, persumably about the atomic center of mass, no? Perhaps I should think of this as "the nonzero q# means that the wave function will collapse to give a measurement of the angular momentum that is either +1 or -1 times a quantity (its sign will be uncertain)." – Buck Thorn Dec 16 '19 at 9:59

Velocity is an observable in quantum mechanics. For a massive particle, the velocity operator is the momentum operator divided by the mass. Eigenstates of energy in an atom are not eigenstates of velocity. If you prepare a hydrogen atom in a state of definite energy, then measure the velocity of the electron, you get a random vector that has a certain probability distribution. The mean is zero.

I don't think any of this really contradicts the statements you list from the other places on this site. Those statements may appear to contradict each other, but that's because they're using imprecise language. When you use English to talk about quantum mechanics, it doesn't necessarily fit.

• Well, the average velocity of Earth when sampled randomly is also 0, isn't it? That does not imply that it has no meaning or structure. (One difference may be that Earth's velocity will reveal a pattern if sampled often enough; is that different with an electron, even at high temporal resolutions?) – Peter - Reinstate Monica Dec 16 '19 at 7:19
• @Peter-ReinstateMonica A localized wavepacket orbiting the nucleus can be analyzed, but it is not an anergy eigenstate and it is not a very stable state (radiation will destroy it). – Vladimir F Dec 16 '19 at 7:50
• I believe linear velocity can be seen as a superposition of rotating in two opposite directions (clockwise/counter..). The squared angular momentum operator commutes (as I recall) with the single electron Hamiltonian. And I think your answer, which states explicitly that velocity is measurable, very much contradicts the statement "velocity of electrons has no meaning". – Buck Thorn Dec 16 '19 at 10:06
• @BuckThorn : The process entailed by "Guess what number I'm thinking." is measurable. What is its meaning? – Eric Towers Dec 16 '19 at 14:59
• @EricTowers Don't think I get what you mean. An expectation value of zero is not equivalent to saying something has no velocity. I think it means what this answer suggests, namely that the measured value is maybe bounded by the prepared state but otherwise not predictable. It seems a leap to claim that "velocity of electrons has no meaning" or that "they don't move around at all", particularly if the concept of momentum or kinetic energy seems integral to describing the Hamiltonian of an electron. Maybe it was meant that unlike a vector, it has no well-defined direction or magnitude? – Buck Thorn Dec 16 '19 at 15:55

In chemistry, angular momentum is often zero.

The situation in molecules or solids is quite different from free atoms. Angular momentum is a good quantum number for free atoms because of spherical symmetry. The spherical harmonic functions $$Y_{\ell,m}$$ are solutions of the angular part of the Schrödinger equation. These are the traveling waves where the phase of the wave function propagates around the nucleus.

In molecules, there is no spherical symmetry. The wave functions are often standing waves, where for example a $$p_x$$ orbital is a sum of counterpropagating $$Y_{1,1}$$ and $$Y_{1,-1}$$. One cannot see anything rotating. In the language of solid state physics of magnetism of the $$3d$$ transition metals one says that the angular momentum is quenched.