The electron id usually defined as a -1/2 (spin-down) particle , implying that p and L vectors point in opposite direction. Is that just an arbitrary convention or is that a basic/preferred orentation of S? When electrons are shot by a gun in a tube, is the direction of the spin just random?

In an atom electrons are paired, but is their spin exactly aligned (anti/)parallel with the axis of p?

  • $\begingroup$ what is the p vector? $\endgroup$ – Señor O Dec 22 '17 at 7:27
  • $\begingroup$ @SeñorO, usaually, p is momentum $\endgroup$ – user157860 Dec 22 '17 at 7:46
  • $\begingroup$ electron spin is a quantum effect and should not be visualized classically $\endgroup$ – SAKhan Dec 22 '17 at 7:58
  • $\begingroup$ @SAKhan, even so , even as an intrinsic property, it must have a direction. Can you have angular momentum without a definite direction? If there were no direction, how can you have electrons paired by opposite (up/down) direction? QM can't have it both ways. $\endgroup$ – user157860 Dec 22 '17 at 8:09
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    $\begingroup$ Hi. QM in fact has it all ways in pure systems before some kind of interaction happen, a measurement for example. If you have an isolated electron, then it may be with spin "up" or "down", and linguistically it is in both states- in at least probabilistic understanding . It will definitely be at one of those states after interaction. $\endgroup$ – Constantine Black Dec 22 '17 at 8:49

Yes, of course there is a preferred spin direction, because electrons have a magnetic moment. Any unpaired electrons will align with the local magnetic field (weakly, but electron paramagnetic resonance relies on generating a signal from this). In spectroscopy, the associated energy-level splitting is called the Zeeman effect.

Two electrons in (for instance) a 3S orbital will pair, but in shells that have empty states, electrons (by Hund's rule) will usually align with each other. That alignment, too, is a spin directional preference.

Ferromagnetism and antiferromagnetism are more complicated, but are alignments of electron spins.

  • $\begingroup$ Thanks, can you describe how the (first) electron aligns joining a proton or a Helium ion? $\endgroup$ – user157860 Dec 22 '17 at 17:37
  • $\begingroup$ In the case of a proton (which has its own magnetic moment), the spin-spin interaction causes a hyperfine splitting of the spectrum lines; this is such a small energy difference, it only matters in very cold environments, but space IS cold, and contains hydrogen. There's a good discussion here <feynmanlectures.caltech.edu/III_12.html> $\endgroup$ – Whit3rd Dec 27 '17 at 4:52

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