darkol933 Posted January 8, 2013 Posted January 8, 2013 When 2 hydrogen atoms bond to create H2 molecule, their atomic orbitals interfere to create a molecular orbital. Since hydrogen has 1 electron, there will be 2 electrons in H2 molecule. If they are in the same orbital, I assume they must have anti-parallel spins -- +1/2 and -1/2. So, my question is: ''How does an electron reverse its spin?''. If two hydrogen atoms have electrons with same spin, say +1/2, how does one of them 'reverse'? I thought that there is a conservation of total spin, so I guess that the electron might emit a photon (spin 1)?
swansont Posted January 8, 2013 Posted January 8, 2013 On 1/8/2013 at 1:27 PM, darkol933 said: I thought that there is a conservation of total spin, so I guess that the electron might emit a photon (spin 1)? Bingo. 1
darkol933 Posted January 8, 2013 Author Posted January 8, 2013 Great, thanks. When I think about it, it makes sense. The molecular orbital is at lower energy state than the atomic orbitals alone, so the extra energy is released via photons.
SamBridge Posted January 10, 2013 Posted January 10, 2013 Bingo. I have question: Why does emitting a photon cause an electron's spin to go down by 1? I didn't know spin was a physical spinning that had physical conservation.
swansont Posted January 10, 2013 Posted January 10, 2013 I have question: Why does emitting a photon cause an electron's spin to go down by 1? I didn't know spin was a physical spinning that had physical conservation. Angular momentum is a conserved quantity when there is no external torque, which applies here. In this case it's not a physical spinning, it's intrinsic angular momentum. But it's real — you can cause rotation of macroscopic objects with it http://scienceblogs.com/principles/2010/04/13/measuring-the-angular-momentum/ The electron is spin 1/2, and can be oriented "up" or "down" (that's the z-axis projection of the angular momentum). Flipping from one orientation to the other is a change of 1, which is what you get from a photon absorption or emission. This gives rise to what are called "selection rules" for whether atomic systems can absorb or emit a photon — some can't, and are sometimes called "dark" states. 1
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