>Helium atoms are also smaller than Hydrogen atoms
How is that?
Hydrogen atom is one proton + one electron [1], and "Helium is composed of two electrons bound by the electromagnetic force to a nucleus containing two protons along with either one or two neutrons, depending on the isotope" [2]
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:
How is that?
Hydrogen atom is one proton + one electron [1], and "Helium is composed of two electrons bound by the electromagnetic force to a nucleus containing two protons along with either one or two neutrons, depending on the isotope" [2]
[1] http://en.wikipedia.org/wiki/Hydrogen_atom
[2] http://en.wikipedia.org/wiki/Helium_atom