I've been looking online at multiple resources and seem to be getting conflicting answers. http://www.sciencegeek.net/tables/AtomicRadius.pdf - Where He is slightly smaller than H http://intro.chem.okstate.edu/1314F00/Lecture/Chapter7/ATRADIID.DIR_PICT0003.gif - Where H is smaller than He http://environmentalchemistry.com/yogi/periodic/atomicradius.html [Broken] - Where H is bigger than He I could keep posting more sources but I think you get the gist of it. Also, the numbers from each source tend to vary and is this because of uncertainty or is another value (ex. covalent radius) being measured instead? Any help in discerning which atom, He or H, has the greater atomic radius and a brief explanation would be greatly appreciated!