The single most important piece of scientific literature is, in my opinion, the periodic table. Those who understand what it means, and what it actually implies, have mastered more science than most professors ever will. This may sound like an exaggeration, but come with me and I think that I can prove it to you.Translator, writing for The Daily Kos
The Dailykos!! Whoever "Translator" is (and he reminds me of Derek Lowe), he or she put together a wonderful historical synopsis of the periodic table. I highly recommend reading at least the historical section to get a flavor for what went into the development of the periodic table.
I would add just one thing to Translator's fine discourse, and that is that the modern appearance of the periodic table might lead one to think that there are gaps between elements:
For example, look at the huge gap between H and He, which essentially spans the whole table. Look in particular at the gap on moving from 4 to 5, ie., beryllium (Be) to boron (B). Why aren't there elements filling those gaps? The answer (besides the "cute" one that there are no whole numbers between 1 and 2 nor between 4 and 5) is that those gaps are a consequence of the two dimensional presentation of the table which has become standard. While the table doesn't map anything physical in the sense of a photograph of unseen things, it might be useful to think of the gaps in the periodic table in the same way that certain flat maps depict a spherical earth:
In essence, what the periodic table represents cannot be perfectly rendered in two dimensions!
There have been numerous alternative presentations of the periodic table: spirals, pyramids, spheres, etc, none of which ever caught on as the iconic image above did. Several of the alternatives are presented here.