Here's why:

* Electrons: Electrons are extremely small and have a very low mass. They occupy a large region of space around the nucleus, constantly moving in unpredictable ways.

* Nucleus: The nucleus, containing protons and neutrons, is incredibly dense. Most of the atom's mass is concentrated in this tiny region.

* Empty Space: The majority of an atom is actually empty space. While the electrons are constantly in motion within the electron cloud, they occupy a vast volume compared to the nucleus.

Think of it like this: Imagine a football stadium where the nucleus is a marble in the center, and the electrons are like tiny dust particles flying around the whole stadium.

Therefore, the vast majority of the atom's volume is made up of the sparsely populated electron cloud, making it the least dense part.