( ( ( The mass ) ( of ( an atom ) ) ) ( ( is ( called ( the ( atomic mass ) ) ) ) . ) )
(ROOT (S (NP (NP (DT The) (NN mass)) (PP (IN of) (NP (DT an) (NN atom)))) (VP (VBZ is) (VP (VBN called) (NP (DT the) (JJ atomic) (NN mass)))) (. .)))
The mass of an atom is called the atomic mass.
(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))))) (. .)))