Expected Gaussian distribution on screen in Stern-Gerlach Experiment

Question

In the Stern-Gerlach experiment, they used a beam of silver atoms travelling in z direction (which has valence electron in the 5s shell), so the electrons carry zero angular momentum. Now since they have zero angular momentum, their magnetic moment must be zero. But when experiment was done by Stern and Gerlach, they expected a gaussian distribution because the magnetic moments were randomly oriented.

But my question is, even if the magnetic moments are randomly oriented, they're still zero, so if hypothetically there were no spin quantization, then we must get a single intense point on the screen because all the electrons must travel in same direction, undeflected by the spatially varying magnetic filed?

Answer 1

The electrons do not have zero angular momentum; they have zero orbital angular momentum, meaning that the total angular momentum of the electrons (and to a good approximation, the silver atoms) is given by the intrinsic angular momentum (spin) of the electrons, which was assumed to be uniformly distributed across all directions. Thus, the magnetic moment is non-zero and proportional to the electron spin, causing the atoms to spread in the magnetic field.

Answer 2

Stern and Gerlach didn't know that electrons carry spin, so they did in fact think that their intrinsic magnetic moment is 0. But through experiments they still knew that the magnetic moment of a silver atom was non-zero. They attributed this to an orbital angular momentum with quantum number $l=1$, though, so they expected three dots on the screen (one each for the quantum numbers $m_l=-1,0,+1$), not two. So the reason for the classically expected Gaussian distribution at the time was a supposed orbital angular momentum.