Sentence:  When typical reaction cells to which 0.3 of an atmosphere of <WSD>oxygen</WSD> had been added were illuminated , chlorine and phosgene were produced . 
Meaning of the word marked with <WSD> tags:  a nonmetallic bivalent element that is normally a colorless odorless tasteless nonflammable diatomic gas; constitutes 21 percent of the atmosphere by volume; the most abundant element in the earth's crust