Sentence:  Very small concentrations of these hydrides should be detectable ; in interstellar gas , concentrations as low as * * f <WSD>molecules</WSD> / * * f may be sufficient , as compared to the * * f hydrogen atoms * * f required for detection of the 21 - cm line . 
Meaning of the word marked with <WSD> tags:  (physics and chemistry) the simplest structural unit of an element or compound