It happened last year when biology got seriously cellular and molecular.
It's happening again with chemistry: some questions I cannot answer:
Why does the 4s orbital fill before the 3d orbital, anyway?
What is it with Chromium--it has only one electron in its 4s orbital, while both its predecessor Vanadium and its successor Manganese have two?
Similarly, Niobium, Molybdenum, Technetium, Ruthenium, Rhodium, Silver, Platinum, Gold, plus the whole horde of f-orbital filling elements--what's going on with them?
The periodic table in the book says that elements 110 and 111 have only one electron in the 4s orbital. Do we know this from experiment? Do we know this from quantum-mechanical calculation? Or is it just a guess from the behavior of Platinum and Gold?
I'm still further handicapped by the fact that I *never* understood the chemistry of metals at all...