( ( By convention ) ( , ( ( no subscript ) ( ( is ( written ( when ( ( a molecule ) ( contains ( ( only ( one atom ) ) ( of ( an element ) ) ) ) ) ) ) ) . ) ) ) )
(ROOT (S (PP (IN By) (NP (NN convention))) (, ,) (NP (DT no) (NN subscript)) (VP (VBZ is) (VP (VBN written) (SBAR (WHADVP (WRB when)) (S (NP (DT a) (NN molecule)) (VP (VBZ contains) (NP (NP (RB only) (CD one) (NN atom)) (PP (IN of) (NP (DT an) (NN element))))))))) (. .)))
By convention, no subscript is written when a molecule contains only one atom of an element.
(ROOT (S (SBAR (IN If) (S (NP (NP (RB only) (CD one) (NN atom)) (PP (IN of) (NP (DT a) (JJ specific) (NN type)))) (VP (VBZ is) (ADJP (JJ present))))) (NP (DT a) (NN subscript)) (VP (VBZ is) (RB not) (VP (VBN used))) (. .)))