For example, Hydrogen was found by Henry Cavendish. Mr. Cavendish was not a Japanese and I'm pretty sure he won't bother inventing the name for Hydrogen in all the languages in the world, especially in a language that doesn't use the same set of alphabet as the languages he mastered. In fact, he didn't even name it. It was Antoine Lavoisier who gave the name Hydrogen. Google translate translates Hydrogen as 水素 (すいそ).
What I want to know is how the kanji 水素 was chosen to represent Hydrogen? What are the rules that one must follow in assigning kanji to a newly found thing/word? Or one can just assign whatever kanji he wants much like how in English you can just assign whatever name you want to something you found, like is the case with Big Bang being named Big Bang even when it is not an explosion.