In a Chemistry 101 sort of way: Helium has more protons, thus more nuclear charge. Its valence electrons are in the same shell but are being "pulled closer" by

In a Chemistry 101 sort of way: Helium has more protons, thus more nuclear charge. Its valence electrons are in the same shell but are being "pulled closer" by the protons, so the total diameter of nucleus + electron shell is "smaller." The reality is, of course, much more complex. Here's Physicsforum (generally a good source) teaching more: http://www.physicsforums.com/showthread.php?t=306457 http://www.physicsforums.com/showthread.php?t=306457