We just learned about the 2 slit experiment in Quantum Chemistry today, where electrons behave as waves when nobody is looking and behave as particles when they are being observed.

So, what would happen If I were to observe an atom (such that all it's electrons now behave as particles instead of waves) for a long period of time?

I know that real atoms have wave electrons, but when you observe them, don't they "turn" into particle electrons and obey Newtonian law like the 2 slit experiment? So what keeps the electrons from falling into the nucleus when you observe them? I was considering how the earth doesn't "fall" into the sun because of a momentum perpendicular to the centripetal force, but the mass of an electron is so much smaller than the nucelus, I'm not sure it will work.

If anyone can give a simple explanation that would be great, I asked the professor and he only said the the Bohr model was inaccurate and it's difficult to observe electrons.

Thanks in advance.

  • 9
    $\begingroup$ Firstly, electrons don't switch between particles and waves; that's a major misconception. Electron is neither wave nor particle. Waves are not real but are of mathematical construct; they impart probability amplitude of finding the electron at a certain coordinate at a specified time. When you measure, you are collapsing the wavefunction into Dirac-Delta function; even after a minute time-interval, the wavefunction again starts to evolve according to the Schroedinger's equation. Electrons don't move in orbits like solar system; we don't need centripetal force. $\endgroup$
    – user5764
    Mar 31, 2016 at 3:14
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    $\begingroup$ And for why electrons don't fall to nucleus; this can be traced to the uncertainty principle; electrons can't get too localised for then the uncertainty of momentum would be very high- that means there might be chance of the electron having very high kinetic energy and getting ejected. $\endgroup$
    – user5764
    Mar 31, 2016 at 3:24
  • $\begingroup$ We cannot observe the electron without photons to (to see it by human eye) or radiation ( to at least detect it ) and both of these probes interfere with the electron in it's state of the electron gun but not in the ground state in the atom. $\endgroup$
    – M.ghorab
    Mar 31, 2016 at 6:50
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    $\begingroup$ Electrons do fall on the nucleus. But they cannot do anything with the proton. They cannot form a particle like the neutron. The neutron has a mass which is significantly higher than the mass of a proton plus the mass of en electron. So the electron stays as near as possible to the proton, but it cannot do anything with it. $\endgroup$
    – Maurice
    Dec 13, 2019 at 21:02

1 Answer 1


As some commenters have stated, this is a complicated topic, and if you really want to understand it you would probably be best to study quantum mechanics itself. Although it's detailed it's also interesting!

However, as to your question, the first thing to ask is what does it mean for an electron to be "observed"? It can't mean just looking at them with your eyes, because then any matter we looked at would suddenly go crazy when all of its electrons change their behaviour. It also doesn't mean just looking in a normal light microscope, because electrons are too small to see in a light microscope. In fact to really properly "see" an electron, we would need to use very powerful high energy equipment. So high energy in fact, it would give the electron enough energy to fly out of the atom it was attached to. After that it perhaps could well fly off like a "normal particle" electron with a Newtonian path.

Now the obvious question everybody then asks is, is there a way to "look at" the electron bound to an atom without giving it so much energy? And the answer according to physics is no, it is actually impossible, to really "look at" a bound electron enough to make it act like a particle, it always takes a big amount of energy. You could "look at" it just a little bit, just enough to find what energy level it is in for example - but as you probably know this isn't enough to stop it behaving like a spread out wave (atomic orbital).

Now as to your question about why it doesn't fall in to the nucleus, you have actually already answered it. Why would it? The earth doesn't fall in to the sun. You mentioned that electrons are very light, but that doesn't really matter. If we hollowed out the earth to make it much lighter that wouldn't affect its orbit. Now the real reason is a kind of "quantum mechanical" version of this argument that I could explain if you like, but it is complicated and really this is the key reason.


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