( ( The ( atomic ( mass unit ) ) ) ( ( is ( ( defined such ) ( that ( ( ( a ( Carbon atom ) ) ( ( containing ( ( 6 neutrons ) ( in addition ) ) ) ( to ( its ( 6 protons ) ) ) ) ) ( has ( ( a mass ) ( of ( ( exactly ( twelve ( atomic ( mass units ) ) ) ) ( -LRB- ( ( ( ( this ( particular isotope ) ) ( with ( 12 nucleons ) ) ) ( is ( known ( as Carbon-12 ) ) ) ) -RRB- ) ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT The) (JJ atomic) (NN mass) (NN unit)) (VP (VBZ is) (VP (VBN defined) (PP (JJ such)) (SBAR (IN that) (S (NP (NP (DT a) (NNP Carbon) (NN atom)) (VP (VBG containing) (NP (NP (CD 6) (NNS neutrons)) (PP (IN in) (NP (NN addition)))) (PP (TO to) (NP (PRP$ its) (CD 6) (NNS protons))))) (VP (VBZ has) (NP (NP (DT a) (NN mass)) (PP (IN of) (NP (NP (RB exactly) (JJ twelve) (JJ atomic) (NN mass) (NNS units)) (PRN (-LRB- -LRB-) (S (NP (NP (DT this) (JJ particular) (NN isotope)) (PP (IN with) (NP (CD 12) (NNS nucleons)))) (VP (VBZ is) (VP (VBN known) (PP (IN as) (NP (NNS Carbon-12)))))) (-RRB- -RRB-)))))))))) (. .)))
The atomic mass unit is defined such that a Carbon atom containing 6 neutrons in addition to its 6 protons has a mass of exactly twelve atomic mass units (this particular isotope with 12 nucleons is known as Carbon-12).
(ROOT (S (NP (NP (DT A) (NN carbon) (NN atom)) (PP (IN with) (NP (NP (CD 6) (NNS protons)) (CC and) (NP (CD 8) (NNS neutrons))))) (VP (VBZ is) (ADVP (RBR more) (RB specifically)) (VP (VBN known) (PP (IN as) (NP (NN carbon) (CD 14))))) (. .)))
A carbon atom with 6 protons and 8 neutrons is more specifically known as carbon 14.