( ( ( ( The ( total number ) ) ( of ( ( protons and ) neutrons ) ) ) ( in ( ( an ( atom 's ) ) nucleus ) ) ) . )
(ROOT (NP (NP (DT The) (JJ total) (NN number)) (PP (IN of) (NP (NNS protons) (CC and) (NNS neutrons))) (PP (IN in) (NP (NP (DT an) (NN atom) (POS 's)) (NN nucleus))) (. .)))
The total number of protons and neutrons in  an atom's   nucleus.
(ROOT (S (NP (NP (DT The) (NN mass)) (PP (IN of) (NP (NNS atoms)))) (VP (VBZ is) (VP (VBN based) (PP (IN on) (NP (NP (DT the) (NN number)) (PP (IN of) (NP (NNS protons) (CC and) (NNS neutrons))))) (PP (IN in) (NP (NN nucleus))))) (. .)))