Here's why:

* Nucleus: The nucleus is the tiny, dense core of an atom. It contains protons and neutrons, which are much heavier than electrons.

* Protons and Neutrons: Protons and neutrons are the subatomic particles that contribute almost all the mass of an atom.

* Electrons: Electrons are much lighter than protons and neutrons and orbit the nucleus. They contribute very little to the overall mass of the atom.

So, even though an atom has a lot of empty space, the tiny nucleus holds the vast majority of its mass.