It came when the one electron of the hydrogen atom transitioned from the 2P 1/2 energy level to the 2S 1/2 level. (The leftmost number is the principal quantum number, much like the discrete but ...

In the search for sharp answers, the lone electron of the hydrogen atom marked the start and end of the road ... would turn out to perfectly capture the behavior of any number of quantum particles, ...

Everyone at that time imagined the atom as a "plum pudding ... The shells in which electrons orbit have different quantum numbers and hold only certain numbers of electrons -- the first shell ...