Information: Bohr's Solar System Model of the Atom
All the planets are attracted to the sun by gravity. The reason that the Earth doesn't just float out into
space is because it is constantly attracted to the sun. Similarly, the moon is attracted to the Earth by
Earth's gravitational pull. So all of the planets are attracted to the sun but they never collide with the
sun.
Negative charges are attracted to positive ones. Therefore the negative electrons in an atom are
attracted to the positive protons in the nucleus. In the early 1900's scientists were looking for an
explanation to a curious problem with their model of the atom. Why don't atoms collapse? The
negative electrons should collapse into the nucteus due to the attraction between protons and
electrons. Why doesn't this happen? Scientists were at a loss to explain this until Neils Bohr
proposed his "solar system" model of the atom.
Critical Thinking Questions: Bohr's reasoning
1. Consider swinging a rock on the end of a string in large circles. Even though you are constantly
pulling on the string, the rock never collides with your hand. Why is this?
2. Even though the Earth is attracted to the sun by a very strong gravitational pull, what keeps the
Earth from striking the sun?
3. There is a gravitational attraction between the Earth and the moon. Why doesn't the moon ever
strike the Earth?
4. Keeping in mind the planetary motion and attractions between planets and the sun, explain how it
could be possible for electrons to not "collapse" into the nucleus?