The pictures of molecules in Chapters 4 and 5 depict atoms as spherical balls. Maybe you’ve wondered what’s inside those balls. Amazingly, scientists have a pretty good idea. Figure 6.1 depicts a generic atom. It’s a collection of just three species of subatomic particle: “electron”, “protonProton”, and “neutronNeutron”. A compact cluster of protonsProton and neutrons—the “nucleus” of the atom—resides at the atom’s center. The electrons wander throughout a larger region of space surrounding the nucleus, which I’ll call the “electron cloud”. In Chap. 4 , when I spoke of the “diameter of an atom”, I was referring to the approximate diameter of the electron cloud (although the electron cloud need not be spherical).