Chapter 31 - Atomic Physics - Problems and Conceptual Exercises - Page 1111: 2

$3.9Km$

We know that $\frac{d_{nucleus}}{d_{atom}}=\frac{d_{baseball}}{r_{electron }}$ This can be rearranged as: $r_{electron}=\frac{d_{baseball}}{d_{nucleus}}(r_{atom})$ We plug in the known values to obtain: $r_{electron}=\frac{7.3\times 10^{-2}}{1.00\times 10^{-15}}(5.3\times 10^{-11})$ $r_{electron}=3.9Km$