Encyclopedia Britannica explains that a Bohr diagram for the stable ion is a diagram in which the nucleus is placed at the center and electrons orbit the nucleus according to discrete energy quanta. The stability problem for atomic diagrams was solved by Bohr by having electrons orbit in quantized shells.
In the early 20th Century, experiments were conducted to determine the structure of the atom. Rutherford's experiments showed that the previous "plum pudding" model proposed by J. J. Thomson was incorrect. This experiment involved shooting alpha particles (helium nuclei) at a thin sheet of gold foil. If the plum pudding model were correct, these particles should have passed right through the sheet of gold foil. However, the alpha particles bounced back from the gold foil and showed that all of the positive charge of an atom must be condensed into a small nucleus at the center of the atom.
However, Rutherford's initial model was unstable if considered using only classical physics. In classical physics, the orbiting electrons would have to give off energy constantly. Eventually, the electrons would lose so much energy that they would crash into the nucleus. Instead, Bohr theorized that due to quantum effects, electrons could only have certain discrete energy levels and orbits.