( ( If ( there ( is ( ( only ( one atom ) ) ( of ( a ( particular element ) ) ) ) ) ) ) ( , ( ( no subscript ) ( ( is used ) . ) ) ) )
(ROOT (S (SBAR (IN If) (S (NP (EX there)) (VP (VBZ is) (NP (NP (RB only) (CD one) (NN atom)) (PP (IN of) (NP (DT a) (JJ particular) (NN element))))))) (, ,) (NP (DT no) (NN subscript)) (VP (VBZ is) (VP (VBN used))) (. .)))
If there is only one atom of a particular element, no subscript is used.
(ROOT (S (NP (PRP You)) (VP (VBP call) (NP (NP (NNS elements)) (SBAR (WHNP (WDT that)) (S (VP (VBP contain) (NP (NP (JJ only) (NNS atoms)) (PP (IN of) (NP (NP (CD one) (NN type)) (PP (IN of) (NP (NN element) (NN native) (NNS elements))))))))))) (. .)))