I've seen that the work function of a particular metal is always greater than its work function. Is it the case always? If so why?
For example, the first ionization energy of sodium is 5.14 eV/atom(source: http://hyperphysics.phy-astr.gsu.edu/hbase/Tables/photoelec.html) while its work function is just 2.28 eV.(source: http://hyperphysics.phy-astr.gsu.edu/hbase/Chemical/ionize.html)
Thank you very much for your valuable suggestions.