Sentence:  **f appeared to be well suited for the study of these matters , since it is a normal paramagnet , with three <WSD>unpaired</WSD> electrons on the chromium , its crystal structure is very simple , and the unknown position of the hydrogen in the strong **f bond provides structural interest . 
Meaning of the word marked with <WSD> tags:  of the remaining member of a pair, of socks e.g.