Does caesium or francium have a lower ionization energy?

$\ce{Cs}$ and $\ce{Fr}$ are in the same group of lowest first ionization energy, but which element has the lowest ionization energy, and why?





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> which element has the lowest ionization energy?
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The ionization energy for Cs is lower than that of Fr.



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> why?
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The [Wikipedia article on Francium](https://en.wikipedia.org/wiki/Francium) states:



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> Francium has a slightly higher ionization energy than caesium, 392.811(4) kJ/mol as opposed to 375.7041(2) kJ/mol for caesium, as would be expected from relativistic effects, and this would imply that caesium is the less electronegative of the two. 
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That is, relativistic effects are more pronounced for francium than for cesium. A greater positive charge density in the nucleus for the larger atom (and higher atomic number) means higher velocity for (here) the 7s electron, which has a higher relativistic mass and thus stays closer to the nucleus than in the case of Cs (where we're dealing with the 6s electron). 


The relativistic effect manifests in a greater effective electronic mass which in turn decreases the effective atomic radius, which makes it harder to pull off that electron (hence the higher ionization energy). See this [Wikipedia article](https://en.wikipedia.org/wiki/Relativistic_quantum_chemistry) for insights on atoms with a high atomic number and associated electrons with a low principal quantum number, my source for almost all of the answer contained in this post.


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