A really small atom, like hydrogen gas can move through what we think of as solid matter. Hydrogen gas can seep through metal! The world of the atom is weird, the iron atom is still tiny to us but it is 55.8 times heavier than a hydrogen atom.

What messes with your head is that the atomic scale is so different than our general experience. If the nucleus is the size of a pingpong ball the electron is about 500 meters away. But... this is an incomplete analogy, the electron isn't solid, like I said, it is weird.