Question

How did Niels Bohr change the model of the atom?

Answer 1

Niels Bohr change the atomic theory by realizing that the electrons did not crash into the nucleus as would be expected in classical physics. Classical physics says that opposites attract and likes repel, so the negative electrons should be attracted to the positive nucleus. But not so. Why?

Bohr took the ideas of Rutherford (nuclear model), Planck (quanta), Einstein (photoelectric effect and spectroscopy and created the Planetary Model.

Bohr proposed that electrons were in energy levels (ground state) and absorbed photons of certain frequencies to move to a higher energy level (excited state). Then the electrons would release the photons in quantized amounts to return to ground state. The releasing of the photons is responsible for the line spectra given off by elements when excited.

This idea was totally counter-intuitive and required a new physics.

Very good presentation that includes all the necessary information can be on this site http://abyss.uoregon.edu/~js/glossary/bohr_atom.html