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The Periodic Table of Middle-Earth was put together by Emil Johansson — a devoted Tolkien fan and an aspiring chemical engineer. Which this awesome chart makes blazingly obvious.
As you can see, Johansson has replaced the standard chemical elements with characters from Lord of the Rings and The Hobbit.
Each character is organized according to race (including Gothmog, who belongs to no known race) and is given a unique symbol and number. Johansson has even added birth and death dates — and a special symbol to denote wearers of the ring. Via Only a chemistry student would organize Lord of the Rings like this.