dgriffith , 8 hours ago

As I understand it, it’s the quantum part of quantum mechanics.

Electrons can only have fixed energy states, they can only radiate or accept fixed sized packets of energy - a “quantum” of energy. So an electron that is hit with the correct sized quantum of energy can be excited up to the next orbital, and it will emit the same sized packet of energy when it returns to its ground state. So they can’t gradually emit radiation and fall into the nucleus.

Eventually electrons should spontaneously decay but that’s predicted to be in 10 to the power of 40 years or something like that.