when you finish entering in the missing ones, please post it !

it is likely not a bug. Mathematica "buys" data (for example, in previous versions the CRC (chemical rubber company) 5? 57th edition was used)

the data is based on experiments that company knows of (collected). it is not "calculated" is it verified as measured

it means if there is not a "reliable source" confirming the value, Mathematica will return {}

there are many things in Chemistry there are not full tables for. Mathematica used to have "very little" of CRC. it now has much more possibly not all. CRC may not be the fullest compendium of verified experimental data outcomes

I hope that helps.

Wolfram (i am not an employee) values what customers are seeking when using their product. If they think applied science is booming they will add more. They've certainly added allot recently.

Huh. Yeah, EntityValue["Element", "PropertyCanonicalNames"] does return more values. I believe there are around 33 values that EntityValue["Element", "PropertyCanonicalNames"] returns that ElementData["Properties"] does not. And I believe there are 4 values that ElementData["Properties"] returns but EntityValue["Element", "PropertyCanonicalNames"] does not return. Normally, I wouldn't take this behavior too seriously; I only did so because MolarRadioactivity was in the documentation for ElementData["..."]. Most of the 33 previously mentioned canonical names do go through ElementData["..."], but it does through via a network request, which means it somehow uses EntityValue["Element", "..."] in the background (since other ElementData["..."] calls are near instantaneous), I believe. This slows down my script tremendously, which makes such calls impractical. Although I would appreciate if this were more explicit, there is probably some location in the documentation that I'm missing here.