TEACHING ATOMIC WEIGHTS ERNEST A. WILDMAN Earlham College, Richmond, Indiana
This paper outlines Aston's mass-spectrographic pro- recent results in the cases of hydrogen, carbon, nitrocedure for determining the atomic weights of complex gen, and oxygen give substance to this expectation. elements and suggests its suitability for presenfetion In 1927 Aston ( I ) built a new mass-spectrograph with which he can determine isotopic masses with an in the early part of the general chemistry course. accuracy of 1part in 10,000 as compared with about 1 + + + + + + art in 2000 formerlv. With this new instrument he has shown definitily that the mass numbers are URING the past few years F. W. Aston has usually not whole numbers. Nearly all have been modified and extended his massspectrographic found to deviate by a small amount, an amount that procedure in such a manner that it is now a is significant as a direct measure of the "packing very accurate method for determining chemical atomic dect," the standard for reference being the oxygen weights. In addition it is the simplest method in isotope of mass 16. theory and it is, consequently, well adapted for early In 1930 Aston (2) devised a photometric method for presentation in the general chemistry course. The measuring the intensity of the lines due to each isotope, traditional method of Cannizzaro has had to wait the accuracy of which is about 1 part in 1000. This upon the development of a variety of other subjects gives a direct method of determining the percentage before it could be explained. This meant that the of each isotope of an element, which, therefore, supstudent has been taught to use atomic weights in plants the former method of calculating it from the calculations long before he could be told how they isotopic masses and the chemical atomic weight (in were obtained, and even then, in the writer's experience, the case of elements with two isotopes only). only a small minority of the class has understood the It is obvious that the combination of an accurate method. The result is that, for a number of students, method of determining isotopic masses and an accurate atomic weights have always remained in the class method of obtaining the percentage of each isotope of fanciful speculations. gives data for calculating the atomic weight of an In the fall of 1931 the writer presented the Aston element. This may be illustrated by Aston's results method during the second week of the course and left for chromium (3). He found four isotopes with integer the traditional method until the latter part of the year. mass numbers of 52, 53, 50, and 54 in the relative The results were sufficientlysatisfactory to lead to the abundance of 81.6, 10.4, 4.9, and 3.10/0, respectively. decision to continue such an arrangement in the future. The accurately determined mass of the "52" isotope All chemists are familiar with the significance of the is 51.948. This is 0.052 less than the integer 52, the mass-spectrograph as an instrument for determining 2 = 0.001 part, which is the the existence of isotopes and their masses. During decrease in mass being E 69 "the years from 1919 to the present its application to "packing fraction," that is, the divergence from the the analysis of nearly two-thirds of the elements has whole-number rule divided by the atomic mass. Its been a notable achievement. The information ob- significance is that it is a measure of the gain or loss of tained by its use and to a smaller extent from band mass per proton when the nuclear packing is changed spectra and the radioactive disintegration series leave from that of 0'' to that of the atom being examined. us a t present without data for twenty-one elements Such data are especially important in the investigation only, including the rare earths, a few other rare ele- of nuclear structure. ments, and gold and platinum.* The 6rst forty-two The same packing fraction may safely be applied elements without exception have been examined. to the other three isotopes and so the corresponding They account for eighty-five mass numbers ranging from 1 to 100. Of the two hundred thirty-eight in- amount may be deducted from the weighted average teger mass numbers possible between hydrogen and mass as shah below: uranium about two-thirds are now assigned to some Wholc Numbn Pnc~xtngc Part of Atomic kind of atom. Refinements in technic will probably of Each W&hl Due lo E o d Designorion of Ixmoic Marsel ComPonrlll CornPam increase this number within the next few years. The
D
F. ALLISONAND E. J. MURPHY, inPhys. Reu..36,1097(1930). reported the use of the magneto-optic method for determlmng the number of i s o t o ~ e sof Au. Pd. Pt, Rh. Ru. Ta. TI. and Th. They found three isotopes for l'a, whereas Aston-.Velure-. 130, 130 (1932)-found that l'a appears to be a simple element within the limiti of accuracy of the present masspectrograph.
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. . . . .. . . .. 52.088 0.052 Atomic weight on b d n of Om.. ..................... 52.016 Weighted avuage mass.. ... . ... .. ... ... . L u o the amount correspondingto the paeking fraction..
This value would stand as it is if it were not for the fact that, unfortunately, the chemical atomic weights have been referred to the oxygen isotopic mixture as the standard whereas Aston's results are determined with respect to the "16" isotope. This means that the chemical atomic weights are smaller by an amount corresponding to the amounts of 0" and 0'' in the natural mixture, an amount estimated by Mecke and Childs (12) as 2.2 parts in 10,000 and confirmed by Aston (13) since the publication of his data on chromium (3). Upon applying the correction the result is 52.0046 which agrees very closely with the accepted weight of 52.01 as determined by chemical methods. For the usual chemical purposes the transfer to the traditional chemical standard has little practical significance, but it is of consequence to physicists and it has a theoretical importance in the teaching of chemistry. The simplicity of this method and of the calculation involved is emphasized by contrasting it with such a scheme as that outlined in Table 1, which summarizes the many steps that the student must keep in mind in order to understand fully the Cannizzaro procedure. Little wonder that most students do not grasp it! TABLE 1 Molecular weights of gases and
volatile ~brtanees G . M. V.
I
I
Approrimate
%tomr
viously depends upon the determination of the relative proportions of the isotopes of oxygen. Consequently it will undergo change as these ratios become more accurately known. The first factor, 1 part in 10,000, was suggested by Birge (11). The factor used during most of the past year is that of Nand6 (4). More recently Mecke and Childs (12) have obtained a different ratio which has been confirmed by Aston (13) by the mass-spectrographic method and which leads t o the factor 2.2 parts in 10,000. The question of changing the standard for chemical atomic weights, because of the present unsatisfactory situation, has received a large amount of attention (14) but it appears that for the present, at least, chemists will continue to use the old standard, upon which 016 = 15.9965, and physicists will use 0 1 6 = 16. It is a curious fact that the two elements that have been relied upon from early times as atomic weight standards have both been shown to be isotopic within the last three years although they had been thought to be simple since the first discovery of isotopes. The following is an example of the kind of presentation of this subject that the writer suggests is suitable for elementary college students. In his own classes the students have previously been made acquainted with electrons, protons, electric discharges through gases, and atomic structure in an introductory way. They have not been told about isotopes. The new method of atomic weight determination is a logical part of the discussion of this subject.
weights
Valenee
SAMPLE PRESENTATION
Isotopes.-The smallest atoms in both size and Percentage composition j weight are hydrogen atoms. They consist of just one weights Quowirarim A m l y ~ s proton and one electron each. The largest known atoms Combining weights.. . ... . . are those of uranium which contain 238 protons and Aston's determinations are receiving favorable at- 238 electrons. These facts suggest that there may be tention on the part of the committees on atomic weights. 238 kinds of atoms, each one differingfrom its numerical In 1929 the American committee (6) changed the value predecessor or successor by one pair of electrical units. for hydrogen to 1.0078 (previously 1.008) largely Perhaps this is true although not quite all of the possiupon the evidence of the mass-spectrograph. In the bilities have been discovered. But, as has already same year the English committee's table (7) contained been stated, there are 92 elements. The reason for the Aston's results for nine elements, all of which were apparent discrepancy is that in a great many cases at that time thought to be simple. In 1930 the Ameri- atoms with different weights are grouped into families can committee (8) lowered the atomic weight of arsenic that have one set of chemical properties. The word so as to conform to a new chemical determination "element" is used for the family as a whole. If one that was in close agreement with the mass-spectro- speaks of a single member he uses the term "isotope." Some elements seem to he represented by only one graph result. In 1931 the first report of the Committee on Atomic Weights of the International Union of isotope. They are sometimes called "pure" or "simple" Chemistry (9) included the detailed results of Aston's elements. I t may he, however, that we consider them determinations in the case of seven complex elements, so merely because of the absence of adequate informaalthough they were not included in the table. I t tion about them. The recent discovery of isotopes called attention to the discrepancies shown by him of oxygen, nitrogen, and carbon, after these elements to exist in the accepted weights for krypton and xenon. had been thought to be simple for a long time, gives A year later the same committee (10) changed the weight to this possibility. The Structure of Isotopes.-The chemical properties values in the table for these two elements in view of Aston's findings and also of the results of other physical of elements depend upon their atomic numbers. Consequently, all of the isotopes of an element must have methods. The factor which should be used in converting mass- the same atomic number. As long as this number spectrograph values to the chemical standard ob- remains constant the size of the nuclei of the atoms
1
exact
may be changed without altering the chemical properties. The way in which this is possible is evident from the accompanying diagrams of the isotopes of zinc and copper.
protons in atoms varies a small amount with each element from that which they have in oxygen. This fact has been only recently proved by means of highly accurate determinations made by Aston. He has found that the mass of the "52" isotope of chromium, for example, is 51.948. The difference is attributed to the "packing fraction." Since the other isotopes of chromium probably have about the same packing fraction we may correct the weighted average mass by subtracting (adding, in the case of some elements) this proportionate part. This value would stand as i t is were i t not for the fact that the old method of determining atomic weights, a method that depends upon the chemical properties of oxygen, referred them to the oxygen isotopic mixture as the standard, whereas Aston's results are relative to its " 16" isotope. On this account the "chemical" atomic weights are smaller than his by an amount corresponding to the concentration of 0'' and 0 1 8 in the natural mixtnre. This causes a decrease of 2.2 parts in 10,000 (0.00022 part) and gives the result 52.0046. The best value that has been obtained by the old method is 52.01. This method of determining atomic weights is gradually replacing the one that was developed during the last century, but as yet most of the values that are in use were obtained by the old procedure. On account of its complicated nature a description of the latter is deferred until later in the course.
The Determination of Isotopes.--Since the isotopes of an element have identical chemical properties they cannot be distiupuished or se~aratedfrom each other by any chemical means. pLysical methods are apLITERATURE CITED plicable, however, since the isotopes differ in their ASTON, new mass.spectropph and the whole number relative masses. Several such methods have been rule," Proc. Roy. Soc., 115A, 487-514 (June, 1927); "Atoms and their packing fractions." Nature, 120, applied to a number of elements but only partial 956-9 (Dec.. 1927). separations have been accomplished in all cases except (2) ASTON, photometry of mass.spectra and the atomic by the use of the "mass-spectrograph," a method of weights of krypton, xenon, and mercury," Proc. Roy. Sac., 126A, 511-25 (Dec., 1930). "positive-ray analysis" that has been perfected by (3) ASTON, "Constitution of chromium," Nature, 126, 200 Aston of Cambridge University. It makes use of a (Aug., 1930). (4) NAUDB,"The isotopes of nitrogen, mass 15, and oxygen. modification of the Crookes tube which is so arranged mass 18 and 17, and their abundances," Phys. RN., 36, that the positive ions are sorted out from each other 344 ( p y ! 1930). by means of electrically charged plates. The results (5) ASTON, u m t of atomic weight," Nature, 126, 953 (DC, 1930). are such that one can obtain the relative masses of the (6) BAXTER."Thirty-fifth annual report of the committee on isotopes and also the relative amounts of the isotopes atomic weights. Determinations published during of each mass. A more detailed description occurs 1928," 1.Am. Chem. Soc., 51, 652 (Mar., 1929). (7) "Revised-!ape of atomic weights for 1929," J. C h m . Soc.. later. I Y Z Y , ZlO. Atomic Weights.-These data make it possible to (8) BBTER,y-hirty.sixth annual of the committee on atomic weights. Determinations published during 1929;' calculate the weighted auerage mass* of the mixture of J. Am. Chem. Soc., 52, 862 (Mar., 1930). isotopes that we call the "chemical" element. This (9) BAxTER, Cunr% H6NIcs-m, LE BEAU, AND MEYER, weighted average mass has for many years been called "First report of the committee on atomic weights of the the "atomic weight," although it is onlv verv recentlv International .-Union of Chemistry;' iM., 53, 1638 (May, 1W1). that this metho;. of obtain& it has come-into use. (lo) Bmn, c u m , H~NIGSCHMID. LE BEAU,AND MEYBR, "Second report of the committee on atomic weights of the The data and calculation are shown in the table (the International 7 n=o\ Union of Chemistry." ihid., 54, 1272 (Apr., one appearing earlier in this article). The masses of the isotopes have been given usually (11) BIRGE,"Probable values of the general physical constants," Phys. Rm. Suppl., 1, 69 (Jan., 1929); "The atomic as whole numbers. These numbers are ratios to the weights of hydrogen and helium," Phys. %., 55, 1015 arbitrary value 16 for the mass of an oxygen atom. (Apr., 1930). For some unknown reason the mass of the constituent (12) ME- AND CHILDS,"Das Atomgewicht des Sauerstoffs,"
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'0"').
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A simple average would s d c e if the isotopes were present in the same amounts. Since they are not, the m a s of each must be weighted by multiplying b y a figure that represents the amount of each, as in the table.
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2. Physik, 68, 362-77 (Jan., 1931). Also ref. (lo), p. ""* 1'10.
(13) ASTON, "Mass-spectra of helium and oxygen," 130, 21 (July, 1932). (14) See ref. (lo). p. 1278.
Nature.
