Periodic puzzle

I'm drinking my coffee from this mug todayLast night's episode of Endeavour, the prequel to Inspector Morse, in which we see a young Detective Constable Morse learning his trade, featured one of those fiendishly complex puzzle-based clues that I am sure real-world detectives never come across (but are still fun for the viewer).
Morse spotted that the set of hymn numbers on a hymn board in a church (which we had earlier seen the soon-to-be-murdered vicar putting up) were strange. The numbers were 74, 17, 18, 19. I was slightly pleased with myself to spot that this was an unusual collection of numbers and probably meant something, but kicked myself for not spotting the clues the writer had carefully provided us for doing the decoding.
We knew that the vicar loved puzzles, had been a cryptographer during the war, and previously had been a chemist - there was even a framed periodic table on the wall of his house.
What Morse spotted, but I kicked myself for not doing, was that if you write out the chemical symbols of the elements with the atomic numbers the vicar put up on the board you get:
W ClArK
And low and behold, the murderer was one W. Clark Esq. Clever, eh?
What struck me since is that I could not do the same for myself. I could do a rather mangled B ClErGe, which might give you a clue, but without an E or a G, it's a bit of a mess. And that led me on to wonder just what the people who devised the chemical symbols were smoking (or inhaling in their fume cupboards). 
It all starts well with a simple rule that seems to be 'use a single letter for the first instance and a two letter variant for subsequent ones.' So in the first couple of rows of the table we get the single letters H, B, C, N, O and F, with He and Be for the next instances. But why is lithium, the first L, Li instead of L? Why is magnesium, the first M, Mg instead of M? You might assume that they decided not to use any more single letter names. Only we later come across P, S, K, V, Y, I (not even the first I) and W.
It's totally bonkers.