Here's why:

* Nucleus: The nucleus, containing protons and neutrons, is incredibly dense and tiny compared to the rest of the atom. It accounts for almost all of an atom's mass but occupies a very small fraction of its volume.

* Electron Cloud: Electrons, which are much lighter than protons and neutrons, occupy a much larger space surrounding the nucleus. They are not confined to specific orbits but rather exist in a cloud of probability, meaning their exact location at any given moment is uncertain.

Think of it like this: Imagine a tiny marble (the nucleus) sitting in the center of a vast, mostly empty room (the electron cloud). The marble is the heaviest part, but the room is where most of the space is.