Question:
Two electrons with the same spin are not allowed to occupy the
same orbital. How does one electron "know" what the other's spin is?
Replies:
This is another of those mysteries involving spin in quantum
mechanics. A single electron can have a spin that points in any direction,
unless there is some external effect (like a magnetic field) that forces an
energy difference between two opposite spin directions (either in the
direction of the field, or opposite to it). When you bring two electrons
together, they have an effect on each other similar to the effect of the
magnetic field - the energy eigenstates either have both electrons spins
pointing in the same direction (a so-called "triplet" state) or they have them
always opposite (a "singlet" state) although the actual direction that both
point (either parallel or opposite to) is again arbitrary. So, there is
actually a direct interaction between electrons that depends on their spins -
this is called the "exchange" interaction -resulting in different energy
states depending on their relative spin orientations.
NEWTON is an electronic community for Science, Math, and Computer Science K-12 Educators, sponsored and operated by Argonne National Laboratory's Educational Programs, Andrew Skipor, Ph.D., Head of Educational Programs.
