Sentence:  Besides the well-known <WSD>hydrogen</WSD> line at 21 cm wavelength , the spectra of extraterrestrial radio sources may contain sharp lines characteristic of other atoms , ions , and small molecules . 
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