Complex sentence: In 1913 Niels Bohr proposed a new model of the atom that included quantized electron orbits: electrons still orbit the nucleus much as planets orbit around the sun, but they are only permitted to inhabit certain orbits, not to orbit at any distance.
Split into simpler sentences: In 1913 Niels Bohr proposed a new model of the atom that included quantized electron orbits. In Bohr's model, electrons still orbit the nucleus much as planets orbit around the sun, but they are only permitted to inhabit certain orbits, not to orbit at any distance.