( ( The ( mass number ) ) ( ( is ( defined ( as ( ( ( ( the sum ) ( of ( the protons ) ) ) and ) ( ( the neutrons ) ( in ( ( the nucleus ) ( of ( an atom ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT The) (JJ mass) (NN number)) (VP (VBZ is) (VP (VBN defined) (PP (IN as) (NP (NP (NP (DT the) (NN sum)) (PP (IN of) (NP (DT the) (NNS protons)))) (CC and) (NP (NP (DT the) (NNS neutrons)) (PP (IN in) (NP (NP (DT the) (NN nucleus)) (PP (IN of) (NP (DT an) (NN atom)))))))))) (. .)))
The mass number is defined as the sum of the protons and the neutrons in the nucleus of an atom.
(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))))) (. .)))