How Small are Atoms?

Why are atoms so small? ... Many examples have been devised to bring this fact home to an audience, none of them more impressive than the one used by Lord Kelvin: Suppose that you could mark the molecules in a glass of water, then pour the contents of the glass into the ocean and stir the latter thoroughly so as to distribute the marked molecules uniformly throughout the seven seas; if you then took a glass of water anywhere out of the ocean, you would find in it about a hundred of your marked molecules.

So small that, if you were to pour a cup of water into the ocean and let it stir in, you could retrieve a cup of water from anywhere in the ocean that would contain 100 molecules from your cup.