Department of Energy Argonne National Laboratory Office of Science NEWTON's Homepage NEWTON's Homepage
NEWTON, Ask A Scientist!
NEWTON Home Page NEWTON Teachers Visit Our Archives Ask A Question How To Ask A Question Question of the Week Our Expert Scientists Volunteer at NEWTON! Frequently Asked Questions Referencing NEWTON About NEWTON About Ask A Scientist Education At Argonne Exclusion Principle
Name: Unknown
Status: N/A
Age: N/A
Location: N/A
Country: N/A
Date: Around 1993

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?

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.

A. Smith

Click here to return to the Physics Archives

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.

For assistance with NEWTON contact a System Operator (, or at Argonne's Educational Programs

Educational Programs
Building 360
9700 S. Cass Ave.
Argonne, Illinois
60439-4845, USA
Update: June 2012
Weclome To Newton

Argonne National Laboratory