# 2.29: Atomic Nucleus

Difficulty Level: At Grade Created by: CK-12
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Practice Atomic Nucleus

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Many people enjoy putting jigsaw puzzles together.  As the different pieces go together, the picture begins to become clearer.  When the puzzle is completed, you see that what had been a confused collection of individual components that made little or no sense by themselves fit together to give a clear picture.

Science works the same way as a jigsaw puzzle.  Different researchers make individual discoveries that answer a specific question or questions.  When enough data are gathered, we get a better understanding of a process or a structure.  The experiments of Crooke, Millikan, Rutherford, and many others gave us pieces of the puzzle that is the atomic nucleus.

### Different Models of the Nucleus

After the electron and proton were discovered, people began to try to build a picture of the atom.  The Bohr model had electrons mixed in with a positive core of some sort that provided electrical neutrality.  Rutherford showed that this model was incomplete.  His picture of the atom involved a small solid core that alpha particles could zoom past with very few collisions.  So the picture of the atom became a little clearer – electrons surrounded a very small core nucleus.  The discovery of the neutron helped fill out the picture even more.  We now have protons and neutrons in a concentrated center of the atom with electrons surrounding the nucleus.

One problem still existed.  We have a number of positively charged protons in the nucleus.  Why don’t they push each other apart? Physicists postulate a strong nuclear force that acts at very short distances. At these distances the attraction between protons is greater than the force causing the positive charges to push each other away. Neutrons are also involved in this process somehow.  So the strong force holds protons together, it holds neutrons together, and it causes protons and neutrons to be attracted to one another.

Video on atomic nucleus:

#### Summary

• Rutherford proposed a model of the atomic nucleus which had a solid core
• There is no good explanation for why the nuclear particles stay together.

#### Practice

1. What is the radius of the nucleus as compared to the radius of the atom?
2. What is the density of the nucleus?

#### Review

1. What was our first model of the atom?
2. How did Rutherford change our thinking about atomic structure?
3. What is our current picture of the atom?
4. Why is the presence of positively charged protons a problem with current models of the atom?
5. How do we explain why the nucleus does not fall apart?

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### Vocabulary Language: English

TermDefinition
nucleus Tiny region in the center of an atom that contains protons and neutrons and makes up almost all of the atom’s mass.

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