text: K.  They're all in the same row, so the one to the left has the biggest radius because there are fewer protons to attract the electrons towards the nucleus.\n\nAtomic radius is how far the electrons are circling around.\n\nEr, no one believes me.  :(\n\nI know the noble gas is supposed to buck the trend, but for some reason, it's listed as having a smaller radius here: http://dl.clackamas.cc.or.us/ch104-07/atomic_size.htm\n\nK has an atomic radius of 2.31 Angstroms or an atomic diameter of 5.54 A, while the others generally go down.\n\nThere's also a graph here that shows the same thing: http://www.webelements.com/webelements/properties/text/image-period-4spd/atomic-radius.html