I often see the charge radius of a proton at .8 * 10^-15m. An acquaintance on a different forum said he would use, if asked to determine the radius of a nucleus, to be: R = R0 * A^(1/3) where R0 = 1.2 * 10 ^ -15 to 1.5 * 10 ^ -15 and A is number of nucleons. He cited the sources: Principles of Modern Physics by Roberth Leighton or Concepts of Modern Physics by Arthur Beuser, or Nuclear Physics by Enrico Fermi. I do see 1.3 * 10^-15 used in the formula above often. Is this the Compton wavelength? If not, then where is this value coming from? If it is the Compton wavelength, I have a follow up question (or 4)... What is the difference between the Compton wavelength and the charge radius? If someone asks me what the diameter of a proton is, what value would I quote? Does it depend on the context of the question and what situation you are applying it to? What is the significance of the value of each? I tried educating myself through reading several wiki articles and arxiv papers. I, somewhat, understand how you arrive at the values of each, but fail to understand the significance of each and how and to what they are applied. I appreciate any insight you can offer and/or online sources you can lead me to.