Sentence:  **f appeared to be well suited for the study of these matters , since it is a normal paramagnet , with three unpaired electrons on the chromium , its crystal structure is very simple , and the unknown position of the <WSD>hydrogen</WSD> in the strong **f bond provides structural interest . 
Meaning of the word marked with <WSD> tags:  a nonmetallic univalent element that is normally a colorless and odorless highly flammable diatomic gas; the simplest and lightest and most abundant element in the universe