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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.

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So little is said about this process by Wikipedia's article on radium that one is led to wonder whether the formation of a nitride is really known. Nevertheless, a possible mechanism is suggested by the section that describes known compounds.

Radium bromide ($\ce{RaBr2}$) is also a colorless, luminous compound.[1] In water, it is more soluble than radium chloride. Like radium chloride, crystallization from aqueous solution gives the dihydrate $\ce{RaBr2·2H2O}$, isomorphous with its barium analog. The ionizing radiation emitted by radium bromide excites nitrogen molecules in the air, [emphasis added] making it glow. The alpha particles emitted by radium quickly gain two electrons to become neutral helium, which builds up inside and weakens radium bromide crystals. This effect sometimes causes the crystals to break or even explode.[1]

Thus, possibly, the radioactivity of radium could cause nitrogen molecules from the air to become more reactive (by exciting them), and then the nitrogen would dominate over the less abundant oxygen upon reaction with the electropositive radium metal.

Heads up for readers: The WP article gives a link to Ref. [1], but I am not allowed to access it from my device. Perhaps a reader can remedy this. (And then remove this paragraph.)

Cited reference

  1. Kirby, H. W.; Salutsky, Murrell L. (1964). The Radiochemistry of Radium (PDF), National Academies Press, pp. 4-8.
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