The nucleus is the least dense region of an atom. Electrons are located far from the nuclei of their atoms. Most of the volume of atoms is filled with the particles they contain. The repulsion of protons and neutrons keeps them on opposite ends of atoms.
I gave my students this multiple choice question: A positively-charged hydrogen ion has a much smaller radius than a neutral hydrogen atom. Which best explains this difference?

Only one answer is correct among these 4 choices:
- The nucleus is the least dense region of an atom.
- Electrons are located far from the nuclei of their atoms.
- Most of the volume of atoms is filled with the particles they contain.
- The repulsion of protons and neutrons keeps them on opposite ends of atoms.

Could you tell me which one is correct?

Electrons are located far from the nuclei of their atoms.