Here's why:

* Electrons: Electrons orbit the nucleus of an atom in specific energy levels. They are constantly moving, creating a "cloud" of negative charge around the nucleus.

* Protons and Neutrons: These particles reside in the nucleus of the atom. While they do have some internal motion, they are much more massive than electrons and their movement is relatively limited compared to electrons.

Think of it like a solar system. The sun (nucleus) is relatively still, while the planets (electrons) are constantly orbiting around it.