( ( ( ( ( ( All atoms ) ( of ( an element ) ) ) ( have ( ( an ( identical number ) ) ( of ( protons ( in ( their centers ) ) ) ) ) ) ) but ) ( ( ( the number ) ( of neutrons ) ) ( may ( ( ( vary ( a little ) ) ( from atom ) ) ( to ( atom ( of ( the ( same element ) ) ) ) ) ) ) ) ) . )
(ROOT (S (S (NP (NP (DT All) (NNS atoms)) (PP (IN of) (NP (DT an) (NN element)))) (VP (VBP have) (NP (NP (DT an) (JJ identical) (NN number)) (PP (IN of) (NP (NP (NNS protons)) (PP (IN in) (NP (PRP$ their) (NNS centers)))))))) (CC but) (S (NP (NP (DT the) (NN number)) (PP (IN of) (NP (NNS neutrons)))) (VP (MD may) (VP (VB vary) (NP (DT a) (JJ little)) (PP (IN from) (NP (NN atom))) (PP (TO to) (NP (NP (NN atom)) (PP (IN of) (NP (DT the) (JJ same) (NN element)))))))) (. .)))
All atoms of an element have an identical number of protons in their centers but the number of neutrons may vary a little from atom to atom of the same element.
(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))))))) (. .)))