atomic radius

In desperation (see previous posts), I go directly to the article Periodic Table of the Elements. No answer there, either, but there is a link to "Atomic Radius." There, I am interested to learn that atoms "do behave as if they were spheres with a radius of 30–300 pm" (though their actual shape and size is less definite), with pm being picometers, that is, one one millionth of a millionth of a meter.

Here, of course, is the test of my earlier calculation. I was off by a factor of one million ... well, actually, by 10,000 to 100,000 times ... my estimate was about that much too large - which, actually, isn't too bad!