It is needed though. Every noun in a language that has genders should have a gender indicated in the entry. I would venture a guess that Arsen is also neuter like in Dutch and German. Names of substances tend to be neuter, but not all of them are. Hydrogen is masculine in German and feminine in Dutch, and probably is masculine or feminine in Luxembourgish too.
Alright then, my created entries will undergo some minor edits. Genders are not my strongest point, as I don't seem to use them that accurately in relative pronouns in Dutch. Living near Rotterdam, I tend to use neuter (dat) for relative pronouns over masculine or feminine (die). I didn't even knew that waterstof was feminine... Well, I know -ing words are feminine, but using a neuter relative pronoun is a tendency here. :) Hasn't got much to do with me forgetting the genders of Luxembourgish nouns anyway...
Thanks for your remark anyway, I'll add them... not straight away though, I got lots of different things to do.