Here's why:

* The nucleus: The nucleus, containing protons and neutrons, is incredibly dense and occupies a tiny fraction of the atom's total volume.

* Electrons: Electrons, which are much lighter than protons and neutrons, occupy a much larger volume, but they don't exist in a fixed location. They exist as a cloud of probability surrounding the nucleus. This cloud, called an electron cloud, is what defines the atom's size.

Think of it like this: Imagine a basketball court. The nucleus is like a tiny marble in the center, while the electrons are like tiny, fast-moving particles buzzing around the entire court. The majority of the space is empty, but the electrons are constantly moving within that space.

This is why atoms are mostly empty space, even though they appear solid to us!