( ( ( The number ) ( of ( neutrons ( within ( atoms ( of ( the ( same elements ) ) ) ) ) ) ) ) ( ( can ( ( vary , ) however ) ) . ) )
(ROOT (S (NP (NP (DT The) (NN number)) (PP (IN of) (NP (NP (NNS neutrons)) (PP (IN within) (NP (NP (NNS atoms)) (PP (IN of) (NP (DT the) (JJ same) (NNS elements)))))))) (VP (MD can) (VP (VB vary) (, ,) (ADVP (RB however)))) (. .)))
The number of neutrons within atoms of the same elements can vary, however.
(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))))))) (. .)))