How big are atoms and atomic nuclei?

Atoms are mostly empty space. Atomic nuclei are 10,000 to 100,000 times smaller than an atom. If a lecture room is 20 meters long, then an atom the size of that room would have a nucleus with a diameter of 2 mm to 0.2 mm. Real atoms have diameters of about 0.074 nm for H to 0.524 nm for Cs.