Why the downvotes? That reply might have been ELI5-level, but I am a physicist. Atoms are fuzzy things without a hard interface boundary. At that scale it does

Why the downvotes? That reply might have been ELI5-level, but I am a physicist. Atoms are fuzzy things without a hard interface boundary. At that scale it doesn't really make sense to say they have a "perimeter" at all. The distance at which they repel each other is different from the distance at which they cling together in a hydrogen bond, which is different from the distance between covalently bonded atoms, etc. The electrons themselves roam all over the place, including far outside whatever bond distance you might choose as the size of an atom. In short, there isn't a perimeter which defines "inside" and "outside" of the atom. The "empty space" thing is a bit of a poor analogy anyway. It comes from chemical experiments which showed that the repulsive cross section of an atom when pummeled with zero-charge particles is orders of magnitude smaller than the electric cross section. These are different forces though (strong vs electromagnetic), so it's a bit apples-to-oranges to begin with.