But if the electron radiated, it would lose energy and soon spiral into the nucleus. Fortunately it doesn't. Both Special Relativity and General Relativity were developed during this time.
Electrons can only be in discrete orbits.
A photon can be emitted or absorbed by an atom only when an electron jumps from one orbit to another. The photon energy equals the energy difference between the orbits. QM describes the microscopic world. There are some very non-intuitive things associated with it. Determinism in QM Classical physics is deterministic.