Yominum is an element, symbol Ym.
Spock input the correct molecular formula for yominum sulfide crystals in his training computer after undergoing the reblending of his katra with his body. (Star Trek IV: The Voyage Home)
The element or its isotopes have not been mentioned before or since Star Trek IV. Yominium [sic] is derived from "Nimoy" spelled backwards. (Star Trek Encyclopedia)