For example, according to classical physics, the electrons orbiting the nucleus should lose energy until they spiral down into the center, collapsing the atom. Bohr proposed adding to the model ...

Therefore, electrons cannot be arranged at random, but they must have fixed levels of energy within each type of atom. Bohr's 'solar system' model of the atom is the way that most people think ...

Everyone at that time imagined the atom as a "plum pudding ... planets around the sun. In 1912 Bohr joined Rutherford. He realized that Rutherford's model wasn't quite right.