The element numbers do not refer to the elements' atomic numbers, but
rather to where the elements occur in Tom Lehrer's song "The Elements".  
This is clued in the flavortext by quotations from the song and from the
patter with which Lehrer introduces and interrupts the song (for instance,
Lehrer says "This may prove useful to some of you someday, perhaps, in a
somewhat bizarre set of circumstances"). Thus element number two is
arsenic, number eight is rhenium, and so forth.

If you shade all of the named elements on a periodic table, the result
spells out XXXIX - that is, 39. The 39th element in the song is niobium,
but the flavortext tells you that niobium is not the correct answer; so
the correct answer is the element with atomic number 39, YTTRIUM.