The Wikipedia article states:
Pure radium is silvery-white, but it readily reacts with nitrogen (rather than oxygen) on exposure to air, forming a black surface layer of radium nitride ($\ce{Ra3N2}$).
Why is that?
I was surprised to learn that, given that $\ce{O2}$ is generally much more reactive that $\ce{N2}$:
Dinitrogen is mostly unreactive at room temperature, but it will nevertheless react with lithium metal and some transition metal complexes. This is due to its bonding, which is unique among the diatomic elements at standard conditions in that it has an $\ce{N#N}$ triple bond. Triple bonds have short bond lengths (in this case, $\pu{109.76 pm}$) and high dissociation energies (in this case, $\pu{945.41 kJ/mol}$), and are thus very strong, explaining dinitrogen's chemical inertness.