Niels Bohr
Why doesn't the electron fall into the nucleus?
That time, that place
Rutherford's atom carried two difficulties.
One is that the electron should fall in. By electromagnetic theory a charge moving in a circle radiates energy away and spirals into the center — in well under a hundred-millionth of a second, by calculation.
The other is that each element emits only certain colors. Hydrogen always shows the same few lines. Why those?
Why this question
Bohr decided to tie the two together.
What if the electron cannot take any orbit at all, but only certain permitted ones? Then it cannot go inside the innermost orbit, and so it does not fall.
What was found
And when an electron moves from one orbit to another it gives out, as light, exactly the energy difference between them.
If the orbits are fixed, the differences are fixed; if the differences are fixed, so are the colors. That is why each element shows its own set of lines.
Calculated out, the positions of hydrogen's lines came out exactly right.
What he could not say was why only those orbits are permitted. He simply posited it — a temporary structure bolted together from old physics and a new assumption.
An electron could take any orbit at all
The positions of hydrogen's spectral lines matching calculation exactly
A dozen years later quantum mechanics replaced the makeshift. Bohr founded an institute in Copenhagen that became the center of the field