How come the trend doesn't stand for Barium to Radium

[Ed Aboud]

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.

 

[Redbelly98]

I'd guess it has to do either with the appearance of all those extra elements in the Lanthanide series, or with the appearance of F orbitals, in between the pairs of elements you mention.

But more specifically, why this would cause a higher I.E. I don't know.

 

[chemisttree]

I'd guess it has to do either with the appearance of all those extra elements in the Lanthanide series, or with the appearance of F orbitals, in between the pairs of elements you mention.

But more specifically, why this would cause a higher I.E. I don't know.

It is because f orbitals are less effective at shielding than d orbitals which are less effective than p and ultimately s orbitals. The nuclear charge increases but the shielding effect is lessened resulting in a greater ionization energy.

 

[Ed Aboud]

Ah makes sense now. Thanks for the help.