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About molecular orbitals?


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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)?

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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.

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