How do electrons “move” around the nucleus?


In school, we were/are taught the Bohr model of the atom, that an electron ***orbits*** a nucleus in an elliptical path. Since quantum mechanics entered popular culture, talk has turned to an ***orbital***. Wikipedia defines this as “a mathematical function describing the ***location*** … of an electron in an atom”, I presume in a moment of time. My question is, where do we think the electron will be in the next moment? Will it

(A) Be in an adjacent location? That is to say, does the electrom move in a continuous manner?

(B) Disappear and re-appear in a non-adjacent location? Or

(C) We don’t know?

(If my premise is wrong, please correct me. Otherwise, I hope to steer clear of digressions which don’t answer specifically A, B or C.)

Thanks in advance.

In: 1

Oh boy, this one is going to be confusing but there’s not much we can do about it. Your premise isn’t wrong so much as quantum mechanics left Bohr and his orbitals way behind but they’re still useful for early understanding

Fundamentally you cannot know the location and momentum(speed/direction) of a particle at the same time, finding out one changes the other, so its really hard to say if an electron moves in a continuous manner around a nucleus

The orbitals are instead considered to be probability clouds where you’re likely to find the electron. [Here’s the probability clouds for Hydrogen with different numbers of electrons](, the brighter the color the more likely an electron is to be there. You can see the that P orbitals (2,1,0) and (2,1,1) are very strong in the center of their shape but fade as you go further out because the electron isn’t bound to a specific distance from the nucleus at all times.

What this leaves us with is that A and B (continuous manner or sudden appearance) are indistinguishable based off the rules of the universe. Even knowing that it existed in two nearby spots right after another still doesn’t tell you its the same electron, it could have been moving away when you first measured it but its impossible for you to know what way it was moving.

The ELI5 version: it exists in an “cloud” region that surrounds the nucleus (aka the orbital). Unless it’s involved in a bond (things get more complex), it probably moves about randomly. We can’t directly observe the behavior of subatomic particles, so we can’t know for sure.

It’s sort of everywhere throughout the orbital all at once until it needs to be in a specific location, and then it chooses a location more or less randomly. Probably. There’s still a lot of debate about it.

If that sounds bizarre, unintuitive, and unhelpful… welcome to quantum mechanics! It’s real weird.

The key concept here is the *wave function*, which is the “mathematical function” that you mentioned. It’s a function that describes the relative probabilities of finding the electron in any specific place if you go looking for it. Until you go looking for it, it’s in all of those places in proportion to their weight in the wave function. Once you look for it the wave function *collapses* to a single possible value and that’s where the electron is.

I’ve used the intentionally vague “go looking for it” here because I don’t think it’s entirely clear yet how this interaction works. There are lots of different conjectures to explain it. There almost certainly isn’t anything special about our consciousness or the act of observation that causes the wave function collapse (and it’s possible the collapse isn’t even a collapse but something else entirely) but we don’t yet have a bulletproof answer on how it works.

C) We do not know.

Quantum mechanics is completely agnostic about what particles are “really” doing between observations. We know we can describe it using wave mechanics (the guts of QM), but there are many physical models for what that wave fundamentally is – whether it’s an artifact of many universes interacting with each other, whether there’s an actual physical wave pushing particles around, or whether particles are actually just a certain kind of structure contained in the wave, and the wave is actually the more fundamental object in the universe. Quantum mechanics describes a mathematical object rich in structure, but fundamentally inaccessible to us, so we currently have to choose how to interpret it.

The wiki article is a little bit off. The orbital does not describe the location, it describes the probability of finding the electron at these locations. The shape of the orbital does not change over time, so if the electron is in different places at different times, it is undergoing some mechanics beyond what QM describes.

Electrons do not have a position at most moments. That’s the key idea here: you cannot think of an electron as a tiny billiard ball flying around. It is, fundamentally, a smeared out “cloud” that has only an *average expected* position and not a single well-defined “position it is definitely in”.

The orbitals don’t change over time. So in that sense, the electron cloud isn’t “moving”. But in another sense, you could talk about the average velocity of the electron itself, and that average is not zero – if you measure the electron, it will on average be moving (quite fast, in fact, by conventional standards). In addition to having an indefinite position, the electron also has an indefinite speed.

Yes, this is all quite weird and not at all intuitive. But it’s how the math works out.