( ( The ( simplest atom ) ) ( ( is ( that ( of ( ( hydrogen , ) ( which ( has ( one electron ) ) ) ) ) ) ) ; ) )
(ROOT (S (NP (DT The) (JJS simplest) (NN atom)) (VP (VBZ is) (NP (NP (DT that)) (PP (IN of) (NP (NP (NN hydrogen)) (, ,) (SBAR (WHNP (WDT which)) (S (VP (VBZ has) (NP (CD one) (NN electron))))))))) (: ;)))
The simplest atom is that of hydrogen, which has one electron;
(ROOT (S (NP (EX There)) (VP (VBP are) (NP (NP (CD one) (NNS electrons)) (PP (IN in) (NP (DT the) (NN hydrogen) (NN atom))))) (. .)))