The trend in ionization energies going down a group is that it decreases between each element. How come the trend doesn't stand for Barium to Radium. Barium has an ionization energy of 502 kilojoules per mole but Radium has an ionization energy of 510 kilojoules per mole. Why is this? Thanks for any help. edit: Just noticed it occurs for La to Ac, Mo to W, Tc to Re, Ru to Os, Rh to Ir, Pd to Pt, Ag to Au, Cd to Hg, In to Tl, Sn to Pb as well.