g. Kaptay *, # E/7, 606 Egyetemvaros, Miskolc, Hungary 3515 (Received 24 October 2011; accepted 01 December 2011)

Transcription

1 J o u r n a l o f J Mn Metall Sect B Metall 48 (1) B (2012) M n n g a n d M e t a l l u r g y On the atomic masses (weights?) Of the elements g Kaptay *, # * Bay Zoltan Nonproft Ltd and Unversty of Mskolc, E/7, 606 Egyetemvaros, Mskolc, Hungary 3515 (Receved 24 October 2011; accepted 01 December 2011) Abstract Atomc masses (weghts?) s an essental nformaton for mnng and metallurgy The paper dscusses four subjects around ths problem Frst, the classfcaton of all the elements s suggested nto 4 classes, based on ther sotope features, determnng the accuracy of ther known atomc masses As part of that, the class of elements s dscussed wth uncertan atomc weghts n accordance wth the 2009 IUPAC recommendatons A better (easer to use) format of atomc weghts s presented for ths class of elements Thrd, t s found not nformatve to leave empty spaces nstead of approxmate atomc weghts for elements wth unstable sotopes Fourth, the term atomc weght vs the term atomc mass s dscussed shortly, n agreement wth the SI system of unts and n contrary to the questonable IUPAC convecton Key words: Atomc mass; IUPAC, Atomc weght; Isotopes 1 Introducton In metallurgcal laboratores and plants the weght (mass, m, kg) of chemcal substances are measured for experments and producton In classrooms, techncal reports and scentfc papers the phenomena takng place durng the experments or producton are dscussed n terms of amount of materal (n, mole) The connecton between them s expressed usng atomc masses (M, kg/mol), through the well known equaton: m M n (1) Eq(1) makes the atomc masses of the elements ther most mportant basc property Atomc weghts are relatve values, expressed relatve to the exact (by defnton) mass of 0012 kg of 1 mole of sotope C 12 (see the dscusson of mole [1]) There s a # Correspondng author: kaptay@hotmalcom DOI:102298/JMMB K

2 154 G Kaptay / JMM 48 (1) B (2012) natural desre of all of us to know the atomc masses of all elements wth as hgh accuracy as possble At ths pont t s mportant to note that due to ther relatve nature, atomc weghts are not lmted even by the relatve standard uncertanty ( ) of the Planck constant [1 2], gvng a natural lmt for the defnton of klogram (although today t s stll defned through an nternatonal artfact [1, 3]) Atomc weghts are determned by the followng general equaton: M x M (2) wth x the mole fracton of sotope of the gven element, M s the relatve atomc mass of the sotope of the gven element The atomc mass of the sotopes s manly measured n physcal laboratores, whle sotope fractons are measured by chemsts IUPAC publshes bennal revews on best atomc weghts of the elements In prevous years the 2007 standard data were used [4] Recently, the 2009 standard data have been publshed [5], accompaned by a more popular presentaton [6], ncludng also a tear off page of the most recent perodc table of IUPAC wth the most recent best atomc weghts It s envsoned that ths perodc table wll be the bass of our educaton and practce n the near future Ths paper s wrtten to dscuss and mprove the format of some values gven n ths standard IUPAC table, wthout argung the valdty of the values themselves Also, a 4 level dvson s suggested here for the 112 elements based on the accuracy of the atomc weghts and the reasons behnd 2 On the four classes of elements based on the features of ther sotopes The four classes of elements wll be presented here n order of the decreasng accuracy of ther atomc weghts (see Table 1) The bass for classfcaton s the specal features of the sotopes of dfferent elements 21 Class A In class A there are 22 elements: Be, F, Na, Al, P, Sc, Mn, Co, As, Y, Nb, Rh, I, Cs, Pr, Tb, Ho, Tm, Au, B, Th, Pa The man characterstc feature of ths class of elements s that they have only one known sotope, wth ts mole fracton of 1 [7] That s why the atomc weghts of these elements have the hghest accuracy, not lmted by the accuracy of x measurements Atomc weghts of these elements are known by 7 10 dgts of accuracy and can be consdered as constants of nature The number of elements n ths class s not expected to ncrease In the contrary, t Table 1 Classfcaton of elements accordng to the features of ther sotopes Class Members Characterstc feature Format of M Accurate dgts n M A 22 Only one sotope (2) 7 10 B 52 No varable sotope composton (8) 4 8 C 10 Varable sotope composton ( 5) D 28 No stable sotope (145) 3

3 G Kaptay / JMM 48 (1) B (2012) mght decrease n future, f a second sotope of any of the elements s dscovered 22 Class B In class B there are 52 elements: He, Ne, Mg, Ar, K, Ca, T, V, Cr, Fe, N, Cu, Zn, Ga, Ge, Se, Br, Kr, Rb, Sr, Zr, Mo, Ru, Pd, Ag, Cd, In, Sn, Sb, Te, Xe, Ba, La, Ce, Nd, Sm, Eu, Gd, Dy, Er, Yb, Lu, Hf, Ta, W, Re, Os, Ir, Pt, Hg, Pb, U The man characterstc feature of ths class of elements s that although they have more than one sotope [7], but the mole fracton of those sotopes was not found to vary n dfferent terrestral samples beyond the accuracy of the measurements [5] Thus, the accuracy of the atomc weght depends on the accuraces of measured x and M values That s why the atomc weghts of these elements are known by a less accuracy compared to class A (see Table 1) The atomc weghts of these elements can also be consdered as constants of nature, at least for natural terrestral samples The number of elements n ths class s expected to decrease n the future, as some of the elements are expected to mgrate to class C as the accuracy to measure sotope compostons mproves further, or f a larger varety of terrestral samples are carefully analyzed On the other hand, some elements mght mgrate nto ths class from class A, f new sotopes of some of those elements are dscovered n the future 23 Class C In class C there are 10 elements: H, L, B, C, N, O, S, S, Cl, Tl The man feature of ths class of elements s that they have more than one sotope [7], and the mole fractons of those sotopes were found to vary n dfferent terrestral samples beyond the accuracy of the measurements [5] Ths fact was frst recognzed by IUPAC n ts 2011 publcaton of the 2009 atomc weghts [5 6] It s a very mportant step n understandng nature It means that the atomc weghts of these elements are not constants of nature any more, but depend upon the physcal, chemcal, and nuclear hstory of the sample materal Instead of a gven value for the standard atomc weght [4], IUPAC now suggests to use an nterval of values for the standard atomc weght For example, for L the 2007 standard atomc weght was: M L = 6941(2) g/mol, whle the 2009 standard atomc weght s: M L = [6938; 6997] g/mol It s mportant to understand that the average value of ths nterval (6968 g/mol) does not represent the most probable value n an average natural terrestral sample (6941(2) g/mol) The meanng of the gven nterval s that all natural terrestral samples fall nto ths nterval (to our best knowledge) Ths nterval s dctated not by the accuracy of measurements, rather by nature The present author welcomes ths major step of IUPAC to express the realty of nature n t s new table of atomc weghts However, the present author does not agree wth the format suggested by IUPAC If only the nformaton M L = [6938; 6997] g/mol s gven to the users, an average user wll nevtably use the average value (6968 g/mol) for practcal calculatons, even f IUPAC clearly states that t should not be done [5 6] By dong so, the average user

4 156 G Kaptay / JMM 48 (1) B (2012) wll make an error n the 3 rd dgt compared to the average terrestral sample It should be noted at ths pont that Table 6 of the orgnal IUPAC revew [5] provdes Conventonal atomc weghts 2009 for users needng an atomc weght value for an unspecfed sample However, n the accompanyng publcaton and n ts tear off perodc table ths nformaton s lost [6] Moreover, the table of conventonal atomc weghts [5] loses the very mportant fndng of IUPAC on the varable nature of natural terrestral samples Therefore, herewth the followng format to present the atomc weghts of ths class of elements s suggested (for L, as an 0056 example): M L ( ) 0003 g/mol Here, the best average value 6 s taken from the 2007 IUPAC value, whle the ± ranges are defned n a way to satsfy the 2009 IUPAC values 1 The followng explanatons should accompany atomc weghts gven n the 0056 format of M L 69 41( 2) 0003 g/mol: n general calculatons (carred out for unspecfed samples) the value of 6941(2) g/mol should be used for L; for elements wth ths format the atomc weghts were found to vary from sample to sample beyond the accuracy of measurements; possble nterval of atomc weghts for L s calculated as = 6938 g/mol (mnmum possble value) and = 6997 g/mol (maxmum possble value), gvng the followng nterval of atomc weghts of L n terrestral samples: [6938; 6997] g/mol The number of elements n class C s expected to ncrease by some further elements to be transferred here from class B, wth the ncrease of accuracy of measurements, or f a larger varety of terrestral samples s carefully examned 24 class D In class D there are 28 elements: Tc, Pm, Po, At, Rn, Fr, Ra, Ac, Np, Pu, Am, Cm, Bk, Cf, Es, Fm, Md, No, Lr, Rf, Db, Sg, Bh, Hs, Mt, Ds, Rg, Cn The characterstc feature of these elements s that they do not have stable sotopes [7], and thus they cannot be provded by an atomc weght value characterzng an average natural terrestral sample That s why IUPAC provdes no nformaton at all on the atomc weghts of these elements [4 6] Although the scentfc reasons of ths decson are well understood, t s not practcal from the pont of vew of understandng chemstry and natural scences If we follow ths practce and dssemnate perodc tables wth empty spaces for the atomc weghts of class D elements, more and more people wll ncorrectly suppose that we have no dea what the atomc weght of these elements mght be Ths s certanly not the case In fact, partly stable sotopes are known for each of these elements wth ther measured sotopc masses and half lves [5] The hgher s the half lfe of an sotope, the longer t exsts, thus the hgher s ts molar rato wthn the sotopes of the gven element Thus, Eq(2) can be modfed to estmate the average atomc weght of ths class of elements through the half lves of the sotopes: M M (3) 1 If ths format s accepted, the best average value can be mproved by IUPAC at a later date

5 G Kaptay / JMM 48 (1) B (2012) wth half lfe (s) of sotope of the gven element wth a relatve sotope mass of M The average atomc weghts calculated by Eq(3) are suggested to be ncluded n the table of standard atomc weghts, rounded to 3 dgts and presented n parenthess The parenthess means that the element s not found n natural terrestral samples However, the approxmated value up to 3 dgts of accuracy provdes reasonably accurate nformaton compared to the empty space provded today n standard IUPAC tables For example, for element 112 the followng value s found by Eq(3): M Cn = (285) g/mol To check the estmatng ablty of Eq(3) three A B class elements (Th, Pa, U) are used wth known sotope composton [7], but wth the gven half lves [5] It s found that the values calculated from Eq(3) and rounded to 3 dgts usng data of [5], concde wth ndependent standard values calculated from measured sotope mole fractons [7] and also rounded to 3 dgts (see [5]) Ths confrms the valdty of Eq(3) The number of elements n class D s expected to ncrease n tme, as IUPAC wll recognze more and more elements beyond the present threshold of 112 (most probably all elements wth atomc numbers hgher than 112 have no stable sotopes) In Table 2 the best atomc weghts of all 112 elements are gven The classes A B C D defned above are gven for each element For classes A B the data of [5] are gven For classes C D the above formats are used and the data gven n [4 5] are combned wth Eq(3) to fnd the approprate values Table 2 The atomc weght of elements n a new, suggested format Atomc number Symbol Class Atomc weght, g/mol 1 H C ( ) He B (2) L C ( ) Be A (3) B C ( 7 ) C C ( 8) N C ( 2) O C ( 3) F A (5) 10 Ne B (6) 11 Na A (2) 12 Mg B (6) 13 Al A (8) S C ( 3) P A (2) S C ( 5) Cl C ( 2) Ar B 39948(1) 19 K B (1) 20 Ca B 40078(4) 21 Sc A (6) 22 T B 47867(1) 23 V B (1) 24 Cr B (6) 25 Mn A (5) 26 Fe B 55845(2) 27 Co A (5) 28 N B (4) 29 Cu B 63546(3) 30 Zn B 6538(2) 31 Ga B 69723(1) 32 Ge B 7263(1) 33 As A (2) 34 Se B 7896(3) 35 Br B 79904(1) 36 Kr B 83798(2) table contnued on the next page

6 158 G Kaptay / JMM 48 (1) B (2012) table contnued ftom the prevous page 37 Rb B (3) 38 Sr B 8762(1) 39 Y A (2) 40 Zr B 91224(2) 41 Nb A (2) 42 Mo B 9596(2) 43 Tc D (975) 44 Ru B 10107(2) 45 Rh A (2) 46 Pd B 10642(1) 47 Ag B (2) 48 Cd B (8) 49 In B (3) 50 Sn B (7) 51 Sb B (1) 52 Te B 12760(3) 53 I A (3) 54 Xe B (6) 55 Cs A (2) 56 Ba B (7) 57 La B (7) 58 Ce B (1) 59 Pr A (2) 60 Nd B (3) 61 Pm D (145) 62 Sm B 15036(2) 63 Eu B (1) 64 Gd B 15725(3) 65 Tb A (2) 66 Dy B (1) 67 Ho A (2) 68 Er B (3) 69 Tm A (2) 70 Yb B (5) 71 Lu B (1) 72 Hf B 17849(2) 73 Ta B (2) 74 W B 18384(1) table contnued on the next row table contnued ftom the prevous row 75 Re B (1) 76 Os B 19023(3) 77 Ir B (3) 78 Pt B (9) 79 Au A (4) 80 Hg B 20059(2) 81 Tl C ( 2) Pb B 2072(1) 83 B A (1) 84 Po D (209) 85 At D (210) 86 Rn D (220) 87 Fr D (219) 88 Ra D (226) 89 Ac D (227) 90 Th A (2) 91 Pa A (2) 92 U B (3) 93 Np D (237) 94 Pu D (244) 95 Am D (243) 96 Cm D (247) 97 Bk D (247) 98 Cf D (251) 99 Es D (253) 100 Fm D (257) 101 Md D (259) 102 No D (259) 103 Lr D (256) 104 Rf D (265) 105 Db D (268) 106 Sg D (270) 107 Bh D (269) 108 Hs D (277) 109 Mt D (276) 110 Ds D (281) 111 Rg D (280) 112 Cn D (285)

7 G Kaptay / JMM 48 (1) B (2012) atomc weghts aganst atomc masses Accordng to the SI system of unts [3], the unt of mass s kg (or g), therefore the molar atomc mass has a unt of g/mol On the other hand, weght s dependent on the acceleraton due to gravty, whch s an lldefned quantty even along the surface of the Earth Therefore, atomc masses rather than atomc weghts should be used n all scentfc wrtng In ths paper atomc weghts have been sometmes used to be n agreement wth the present IUPAC wordng [4 6] However, t s suggested here that ths wordng should be changed and publshed by IUPAC, n accordance wth the SI system [4] ME Weser, M Berglund: Atomc weghts of the elements 2007 Pure Appl Chem, 81(11) (200) [5] ME Weser, TB Coplen: Atomc weghts of the elements 2009 Pure Appl Chem, 83 (2) (2011) [6] TB Coplen, NE Holden: Atomc Weghts no longer constants of nature Chem Int, 2011, March Aprl, [7] M Berglund, ME Weser: Isotopc compostons of the elements 2009 Pure Appl Chem, 83 (2) (2011) acknowledgement Ths work was carred out as part of the TAMOP 421B 10/2/KONV project wth support by the European Unon and the European Socal Fund References [1] G Kaptay, On the fve base quanttes of nature and SI (The Internatonal System of Unts), J Mn Metall Sect B Metall 47 (2) B (2011) [2] PJ Mohr, BN Taylor, DB Newell: CODATA recommended values of the fundamental physcal constants: 2006 Rev Modern Phys, (80) (2008) [3] The Internatonal System of Unts (SI), 8th edton, common/pdf/s_brochure_8_enpdf