Atoms of cool hydrogen emit 21-cm radiation when the
A) electrons move from the fourth to the second energy level.
B) electrons move from the second to the fourth energy level.
C) protons change their spin.
D) electrons change their spin.
E) hydrogen atoms add a second electron to form H-.

Question

Quizplus · Accepted Answer

The 21-cm radiation is emitted when the electron in a hydrogen atom changes its spin relative to the proton, transitioning from a parallel to an antiparallel spin state.

Atoms of Cool Hydrogen Emit 21-Cm Radiation When the