On the Name for the Number of Atoms in 12 g of Carbon-12 Rorneu C. Rocha-Filho Universidade Federal de SBo Carlos, 13560 S2o Carlos-SP, Brazil
The number of atoms (No) in a sample of carbon-12 weighing exactly 12 g is quite commonly referred to a s the Avogadro number. The most recently recommended value forNois 6.0221367(36)x loz3(11,where the digits in parentheses are the one-standard-deviation uncertainty in the last digits of this value. Note that No is a dimensionless parameter, that is, it has no units. As Lybeck e t al. (2)have pointed out before, IUPAC does not officially recognize No as the Avogadro number,' and thus No is not listed among the many quantities in IUPAC's most recent edition of its so-called Green Book (3). Many t i e s the expression "Avogadro number" is mistakenly used explicitly o r implicitly, to refer to the Avogadro constant, NA(see, for instance, refs. 4 and 3,the proportionality constant between the amount of substance, n, and the number of entities, N, in a sample (61, that is: (1) N=NAxn As it becomes clear when quantity calculus is applied to this equation, NAhas the unit mol-I. Bewildering as i t may be, a s a consequence of the definition of the SI base unit mole, actually: NA= No mo1- 1 (2) On the other hand, as a consequence of the definnion of the S1 base unlt mole and of the defin~uonof the ourlied atomic mass unit u, No intermediates the relationship between the latter unit and the gram, that is: (3) Ig=Nou or, a s recently recalled by Emiliani (71,NO= 1giu. Confirming IUPAC's nonrecognition of the existence of No, the chairman of IUPAC's Commission on Physicochemical I 'Since this number is not included in IUPAC's Green Book (4, here opted to use the symbol No*fter Lybeck et al. (2). 21nstead of Emiiiani's definition of No as equal to 1 glu (7), 1 rather prefer to use first principles,that is, to defineNo simply as the number of atoms in 12 g of carbon 12.
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Journal of Chemical Education
Symbols, Terminology, and Units;I. Mills, questioned Emiliani's definition2 of No, and restated it a s 1 g/u = NA mol (8).However, considering eq 2 above, clearly there is no difference a t all between the statements of Emiliani (7) and Mills (8). The nonrecognition of No i s only m d e r s t a n d a b l e if IUPAC is trying to avoid the confusion of having the name Avogadro attached to two different but related quantities and has not yet arrived a t another suitable name forN0 (or NA). Hereinafter, considering that IIJPAC already recognizes NAa s the Avogadro constant, I would like to s w--e s t an alternative nam; fur N(,, smce I hrlieve to be pedagogias cally important that NOand .\'A be clearly distinguished different quantities. The actual value ofNo (experimentally determined, see ref 9) i s a consequence of t h e choice of the reference nuclide. Since the beginning of the 60's, when the scales of atomic and nuclidic masses were unified, carbon-12 has been the reference nuclide. The use of this nuclide as reference was independently suggested by t h e American physicist A. 0 . Nier and the Swedish chemist A. Olauder (10). Hence, I would like to suggest that No, the number of atoms in 12 g of carhon-12, be referred to a s the NierOlander number. The adoption of such a name for NOwould make clear to all that No and NA are different quantities. The fact that these quantities would then be unequivocally different would be very helpful from a pedagogical point of new. ~~
Literature Cited 1. Cohen, E. R.;Taylor, B.N. CODATA Bvllolin no. 63; Pernagamon: Oxford. 1538.p 24. 2. Lybeck, L.; Stmmdahl, H.; Tullberg, A. S l u d ~ n t Conepiions i ofAmounl ofSubsfonceondIts SSIUnil the M o k ASubjed Dzdoetic Study; Gothenburg University: MolndalfSwedenl, 1985. 3. Mills, I.; Cviias. T;Homann, K: Kallay, N.; Kuehifsu. KIWAC'c' Qamlillps, Units and Symbols in Physiml Chemistry: Blackwell: Oxfodord, 1988. 4. J. H. M. Thomley: I. R.Johnson. Chsm. hl 199% 12,130. 5. Mdi,R. J. J Chcm. Educ 1989.66. 1064. 6. Roeha-Fiho, R. C. J Chem Educ 1990,67,l39. 7. Emiliani, C. Chem. & Eng News A u g 27,1530. p 3. 6. mill^. I. Chsm. & Eng. News D e 24,1530, p 3. 9. Ref. I, p 13. 10. Kahman,T. P.;Mattauch,J.H. E.: Wapstra.A. H P h y s Today 1959.12.30.
