Here's why:

* The atom is mostly empty space: This is true. The nucleus, containing protons and neutrons, is incredibly small compared to the overall size of the atom. The electrons occupy a much larger volume, orbiting the nucleus.

* The nucleus is dense: Yes, the nucleus is incredibly dense, containing nearly all the mass of the atom in a tiny space.

* The electron cloud: Electrons don't orbit the nucleus in a neat, defined path like planets around a sun. Instead, they exist in a cloud-like region of probability, where their position and momentum are described by quantum mechanics.

In summary: Modern atomic theory describes the atom as a small, dense nucleus surrounded by a diffuse cloud of electrons. This cloud represents the probability of finding electrons in a specific region, not a definite orbit.