Rules and exceptions

We ponder the non-nobility of a Noble Gas.

Our tutoring consultant’s students are now back from Spring Break and facing exams in the coming month.  Many are finding that material from last fall is very fuzzy indeed, which understandably makes them a bit anxious.

This is less a problem with mathematics, which tends to build on earlier work, than with chemistry.  That subject seems to be made up of a myriad of rules-of-thumb to be memorized; at the High School level, the unifying principles can be hard to make out.  And each rule has its exceptions, which must also be memorized.

One of the easier rules, however, deals with the “noble gases,” over there at the right-hand side of the Periodic Table.  They don’t form compounds.  Having full shells of electrons, they have no motivation to share with other atoms (which is the basis of almost all chemistry).  Students have no need to worry if their oxides are acidic or basic, which dissolve in water and which don’t, or what shape their molecules take.

Except that, in the 1960s, several molecules containing the noble gas Xenon and the very reactive Fluorine were prepared.  (No doubt this was very good and useful science, but we suspect the chemists involved of just wanting to prove their teachers wrong.)  Well, all-noble-gases-but-Xenon is easy enough to memorize.  There are a few molecules containing Krypton, but they seem to be very unstable and can be ignored for our purposes.  At the top of the Noble Gases there’s Helium, whose atoms have least motivation of all to form bonds or even clump together.  It doesn’t even freeze at atmospheric pressure, no matter how cold you get it.

Except that a recent paper has reported the detection of the Helium Hydride ion, HeH+.  It’s not likely to cause the rewriting of any High School Chemistry textbooks, however.  The detection was reported by a team of astronomers, and the ion was found in the very high-energy, low-density regions of a planetary nebula.  Indeed, many species that would be considered mistakes if found on a student’s paper exist out in space, and sometimes are important there.  To bring them into an introductory chemistry course would only spread confusion.  Our tutor does hesitate when asked for absolute statements (“Is it always this way?”), since many would be technically wrong, but has taken to qualifying them with, “As far as you need to know now.”  You have to start learning with simplified ideas, not the fully-developed details.

So he will not be telling his students about this Helium compound.  They have enough to manage with the rest of the material forming their chemistry courses.  Except, maybe, for the students curious about the early universe, for helium hydride was the very first molecule ever formed.