( However ( , ( ( ( the number ) ( of ( neutrons ( in ( an atom ) ) ) ) ) ( ( can vary ) . ) ) ) )
(ROOT (S (ADVP (RB However)) (, ,) (NP (NP (DT the) (NN number)) (PP (IN of) (NP (NP (NNS neutrons)) (PP (IN in) (NP (DT an) (NN atom)))))) (VP (MD can) (VP (VB vary))) (. .)))
However, the number of neutrons in an atom can vary.
(ROOT (S (NP (NP (DT The) (NN number)) (PP (IN of) (NP (NNS neutrons)))) (VP (MD can) (VP (VB vary) (PP (IN between) (NP (NP (NNS atoms)) (PP (IN of) (NP (DT the) (JJ same) (NN element))))))) (. .)))