By the 1920's, Bohr's model of the atom was shown to have troubles. Bohr's atom seemed to be too simple to describe the heavier elements. In fact it only worked roughly in these cases. The spectral lines did not appear correct when a strong magnetic field was operating on the atoms.
Bohr and a German physicist, Arnold Sommerfeld changed the original Bohr model to explain these variations. According to the Bohr-Sommerfeld model, not only do electrons travel in certain orbits but the orbits have different shapes and the orbits could tilt in the presence of a magnetic field. Some orbits are circular, others are elliptical and they can even swing back and forth through the nucleus in a straight line.
