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Homework Help: What is spin?

  1. Oct 10, 2005 #1
    In chemistry, we deal with spin to explain why there is a limit to the number of electrons that can occupy an orbital, ie. no two electrons can have the same quantum numbers. We are not required to know much more than that. But I am curious, what is spin? My chemistry book explained an experiment where hydrogen was passed through a magnetic field and half of the atoms were attracted to one side, and the other half where attracted to the other. So does spinning charge induce a magnetic field? Is that why electrons are said to have spin?

    Also, if that is so, and two electrons with opposing spin occupy the same area, should not they attract each other since that have opposite poles?

    And why are spin of particles always simple rational numbers, why are there no 3/12344 spins?
  2. jcsd
  3. Oct 10, 2005 #2


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    (Maybe the admin should move this to QM.)
  4. Oct 10, 2005 #3
    I can say a little about spin...

    At first there was a classical idea about the electron actually spinning I think. You can certainly draw a lot of analogs between classical spinning and the quantum spin, which is why it's called "spin". However, that analogy breaks down and we really can't say that the particles spin. It's better just to consider it as a certain quantum property.

    Spin does yield a certain magnetic moment, which causes particles to behave in certain ways in magnetic fields, like you said.

    I don't know if there is an answer to why certain particles only have certain spin states.
  5. Oct 11, 2005 #4


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    You might be thinking of magnetic monopoles which haven't been discovered yet?
  6. Oct 11, 2005 #5
    Hmm can't edit... oh well...

    Actually, after thinking about it, there are multiple reasons why the spin is always n/2.

    You can simply talk about spin values (lower case cursive L) with respect to the number of states a particle can be in ('m'). From that you can see one reason why all spin states are L= n/2 (they simply derive from how many possible states the particle can be in). (n=0,1,2,3,etc)

    Though there are some more complex quantum derivations.
  7. Oct 12, 2005 #6
    it has to make a full wave , spin is +/- 1/2 because an electron is a fermion. the 1/2 means that in order to return full swing back to their current state they have to make 2 revolutions (2 actual spins) ... now, Pauli exclusion tells us that +/- 1/2 is key because no 2 electrons can have the same quantum numbers, since spin is a quantum number, then we would require a way to differentiate between 2 electrons in with the same n, l and m_l values...

    Now, the following I'm not sure is correct or not, but its how I kinda got the 'jist' of it the first time I was in a room when it was taught (by a very bad teacher) ... the reason WHY one electron takes a +1/2 and the next in the orbital takes (2px for example) takes a -1/2 is because the energy required to have the same magnetic north as the first electron in the orbital makes it impossible for the electron to be in that orbital. Now, I've never confirmed or disproved this, and I kind of just accept the fact that the electron, which doesn't actually have a structure or anything ... , does have 'spin' , because a lot of people that are a lot smarter than me have tried to disprove it, confirm it etc.. and it has held 'firm' ...

    edit: about the nice numbers of spin, you really need to think about the way waves go ... if you had the 3/pi spin or something then it would take multiple movements around a circle just to get back to the original state, the 'nice numbers' can best be seen by looking at standing waves, and 'wrapping' one around a circle .. the circle representing not the e-'s path, but instead more like the % of the path it has gone ....
    Last edited: Oct 12, 2005
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