Sentence:  If the **f bond is linear then there are three reasonable positions for the hydrogen atoms : ( 1 ) The <WSD>hydrogen</WSD> atoms are centered and hence all lie on a sheet midway between the oxygen sheets ; ( 2 ) all hydrogen atoms lie on a sheet , but the sheet is closer to one oxygen sheet than to the other ; ( 3 ) hydrogen atoms are asymmetrically placed , either randomly or in an ordered way , so that some hydrogen atoms are closer to the upper oxygen atoms while others are closer to the lower oxygen atoms . 
Meaning of the word marked with <WSD> tags:  an atom of hydrogen