Ionization energy, what's needed to remove 1 electron from 1 atom, is high for this non-reactive 6-element group
This is because additional electrons in the same shell do not 
substantially... [DOC] [TLE] Chemistry Tutorial : Trends in Ionization Energy - AUS-e-TUTE [PAR] Trends in ionization (ionisation) energy of the elements suitable for high ... is 
measured in kilojoules per mole (kJ mol-1) or electronvolts per atom (eV)*.