( ( ( The number ) ( of ( protons ( in ( an atom ) ) ) ) ) ( ( ( ( is ( ( the ( atomic number ) ) ( of ( the atom ) ) ) ) , ) ( while ( ( ( the sum ) ( of ( ( ( ( the number ) ( of protons ) ) and ) ( ( the number ) ( of ( neutrons ( in ( an atom ) ) ) ) ) ) ) ) ( is ( the ( atomic ( mass number ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NP (DT The) (NN number)) (PP (IN of) (NP (NP (NNS protons)) (PP (IN in) (NP (DT an) (NN atom)))))) (VP (VBZ is) (NP (NP (DT the) (JJ atomic) (NN number)) (PP (IN of) (NP (DT the) (NN atom)))) (, ,) (SBAR (IN while) (S (NP (NP (DT the) (NN sum)) (PP (IN of) (NP (NP (NP (DT the) (NN number)) (PP (IN of) (NP (NNS protons)))) (CC and) (NP (NP (DT the) (NN number)) (PP (IN of) (NP (NP (NNS neutrons)) (PP (IN in) (NP (DT an) (NN atom))))))))) (VP (VBZ is) (NP (DT the) (JJ atomic) (NN mass) (NN number)))))) (. .)))
The number of protons in an atom is the atomic number of the atom, while the sum of the number of protons and the number of neutrons in an atom is the atomic mass number.
(ROOT (S (NP (NP (DT The) (NN sum)) (PP (IN of) (NP (NP (DT the) (NNS masses)) (PP (IN of) (NP (NP (DT the) (NNS atoms)) (PP (IN in) (NP (DT the) (NN formula)))))))) (VP (VBZ is) (VP (VBN referred) (PP (TO to) (NP (RB as) (JJ molecular) (NN mass))))) (. .)))