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After reading this section you will be able to do the following:

  • Define the terms "strong force" and "binding energy."
  • Explain what is meant by a stable atom and and an unstable atom.

How do atomic particles interact?

There are forces within the atom that account for the behavior of the protons, neutrons, and electrons. Without these forces, an atom could not stay together. Recall that protons have a positive charge, electrons a negative charge, and neutrons are neutral. According to the laws of physics, like charges repel each other and unlike charges attract each other.

So what makes the protons stay together in an atom?

A force called the strong force opposes and overcomes the force of repulsion between the protons and holds the nucleus together. The energy associated with the strong force is called the binding energy. The electrons are kept in orbit around the nucleus because there is an electromagnetic field of attraction between the positive charge of the protons and the negative charge of the electrons.

Does the nucleus of an atom ever lose particles?

In some atoms, the binding energy is great enough to hold the nucleus together. The nucleus of this kind of atom is said to be stable. In some atoms the binding energy is not strong enough to hold the nucleus together, and the nuclei of these atoms are said to be unstable. Unstable atoms will lose neutrons and protons as they attempt to become stable.


  1. Electromagnetic fields cause like charges to repel each other and unlike charges to attract each other.
  2. The protons stay together in the nucleus because the strong force opposes and overcomes the forces of repulsion from the electromagnetic field.
  3. Binding energy is the energy that is associated with the strong force, and this energy holds the nucleus together.
  4. A stable atom is an atom that has enough binding energy to hold the nucleus together permanently.
  5. An unstable atom does not have enough binding energy to hold the nucleus together permanently and is called a radioactive atom