( ( ( The ( atomic number ) ) ( of ( ( an element ) ( -LRB- ( A -RRB- ) ) ) ) ) ( ( ( ( equals ( ( the number ) ( of ( protons ( in ( its nucleus ) ) ) ) ) ) , ) ( while ( ( ( the ( mass number ) ) ( -LRB- ( M -RRB- ) ) ) ( is ( ( the number ) ( of ( ( protons and ) neutrons ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NP (DT The) (JJ atomic) (NN number)) (PP (IN of) (NP (NP (DT an) (NN element)) (PRN (-LRB- -LRB-) (NP (NNP A)) (-RRB- -RRB-))))) (VP (VBZ equals) (NP (NP (DT the) (NN number)) (PP (IN of) (NP (NP (NNS protons)) (PP (IN in) (NP (PRP$ its) (NN nucleus)))))) (, ,) (SBAR (IN while) (S (NP (NP (DT the) (JJ mass) (NN number)) (PRN (-LRB- -LRB-) (NP (NNP M)) (-RRB- -RRB-))) (VP (VBZ is) (NP (NP (DT the) (NN number)) (PP (IN of) (NP (NNS protons) (CC and) (NNS neutrons)))))))) (. .)))
The atomic number of an element (A) equals the number of protons in its nucleus, while the mass number (M) is the number of protons and neutrons.
(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))))) (. .)))