Electron Spin and the Stern-Gerlach Experiment
A beam of silver atoms splits into exactly two in a non-uniform magnetic field, which no orbital motion can explain. Electrons carry an intrinsic angular momentum with only two possible projections.
What a learner can do afterwards
- Describes the experiment and why two beams rule out a classical magnetic moment
- States the allowed spin projections and the associated magnetic moment
- Explains fine structure as a coupling between spin and orbital motion
1 · Read
Stern and Gerlach sent silver atoms through a nonuniform magnet and got exactly two beams. A classical magnet could tilt at any angle and would smear into a band, so two sharp spots mean the underlying magnetism has only two settings. No orbital motion can explain that pattern: orbital projections come in odd numbers, 2l plus 1, and silver's outer electron has l equals 0, which alone would give one undeflected beam.
The explanation is electron spin, an intrinsic angular momentum with quantum number one half and only two projections: plus or minus h bar over 2, called up and down. Its magnetic moment steered the two beams apart, and it joins the three hydrogen numbers as a fourth degree of freedom.
Silver works because each atom carries one unpaired outer electron whose spin decides the deflection. Up goes one way and down goes the other, painting two spots. A classical spray of tilts would have painted a smear instead.
Once the electron also orbits, with l above zero, spin does not act alone: its magnetism pushes and pulls on the orbital magnetism, a coupling that shifts levels by small amounts called fine structure. Lines that looked single turn out to be close groups, often pairs, split by that handshake.
Two beams prove two settings, and spin names the extra magnet.
Readings written for this dataset.
2 · Watch
Take it off screen
Where it sits
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.