( ( atom ( ( The ( basic unit ) ) ( of ( a ( chemical element ) ) ) ) ) . )
(ROOT (NP (NP (NN atom)) (NP (NP (DT The) (JJ basic) (NN unit)) (PP (IN of) (NP (DT a) (NN chemical) (NN element)))) (. .)))
atom   The basic unit of a chemical element.
(ROOT (S (S (NP (NN Element)) (VP (VBZ is) (NP (NP (DT the) (JJ basic) (NN chemical) (NN building) (NN block)) (PP (IN of) (NP (NN matter)))))) (: ;) (S (NP (PRP it)) (VP (VBZ is) (NP (DT the) (JJS simplest) (NN chemical) (NN substance)))) (. .)))