Why could electrons occupy only certain orbits in the Bohr model?

In the Bohr model, electrons are restricted to specific, discrete orbits because Niels Bohr postulated that their angular momentum is quantized—meaning it can only take on certain fixed values.1

This approach was a necessary departure from classical physics. In the earlier Rutherford model, electrons orbiting a nucleus would continuously emit electromagnetic radiation, causing them to lose energy and spiral into the nucleus, rendering the atom unstable. By requiring electrons to move in orbits of fixed size and energy, Bohr ensured the stability of the atom.12

These allowed orbits are determined by Planck’s constant, and an electron can only transition between these states by absorbing or emitting energy in discrete quanta. This quantization successfully accounted for the discrete wavelengths observed in the emission spectrum of hydrogen.123