A COMPARISON of SOME PHYSICAL and CHEMICAL

ATOMIC WEIGHTS* GREGORY PAUL BAXTER

Harvard University, Cambridge, Massachusetts

I HAVE hoped that tonight you might be interested in a comparison of some of the results obtained by chemical means and by the mass spectrograph for

certain atomic weights, especially those of the mixtures of lead isotopes which constitute the end-products of the different radioactive disintegration series. In the initial enthusiasm over the development of the mass spectrograph in the skilful hands of Aston it was a common belief that the day of the atomic weight chem- ist was over, that here was a method by which not only the relative masses of individual isotopes could he ac- curately measured, but also .their proportions. With- out question this method has great possibilities, and in the course of time may supplant the chemical method. At the present time the two methods supple- ment one another, in that where lack of reasonable agreement exists, there seems to be good reason to he- lieve that one or the other method is a t fault.

In the mass spectrograph charged atoms a t high velocity are electrically and magnetically deflected so that they are focused on a photographic plate, on a scale linear with reference to mass. Measurement of the relative distances between lines then gives directly the relation between masses. These are ordinarily referred to the most abundant oxygen isotope, of mass 16. On this scale, for the atoms between 16 and about 200 in mass, the values found by Astou are slightly less than integral; for those below 16 the va lvs are greater, and for those above 200 probably greater than integral. But since the element oxygen consists of a t least three isotopes of masses 16, 17, and 18, the physical scale must be corrected to the chemical scale for comparison. The factor by which the physical values usually are divided, which depends on the determination of the proportions of the oxygen isotopes by Mecke and Childs, is 1.00022, although there is still some uncertainty in this value.

Physical Physical 0'" 16.0000 0 = 16.0000

Helium Fluorine Pbo~phoruo Scandium Arsenic lodine Cedum Bromine

It seems to me that this physical method of determin- ing atomic weights is likely to he most satisfactory. in the case of elements which give only a single line in the mass spectrograph, that is, which are presumably simple elements, consisting of only a single variety of atom. The correspondence between values obtained by the chemical and physical methods for elements now sup- posed to be simple is shown in the table below. On the whole the agreement between the two sets of results isveryreassuring, especially in view of the fact that the accuracy claimed by Aston for the mass spectrograph is not over 0.01 per cent.

In the case of complex elements a further uncertainty is introduced in that the proportions of the isotopes must he estimated either. photometrically from the intensities of the mass-spectrum lines, or by other physi- cal means. While somewhat less satisfactory than with simple elements, in general there is fairly close agree- ment between the physical and chemical atomic weights of complex elements, and it is probable that this situa- tion will improve with time. In.+he case of bromine, for instance, the two isotopes'seem to be present in exactly equal proportions and the physical and chemical atomic weights are very nearly identical.

I should now like to call your attention to what are sometimes called radiogenic leads, that is, the various kinds of lead that are produced a s Sqd-products in the radioactive decay of uranium and thorium. These a t present are supposed to consist of RaG and AcD, produced from two isotopes of uranium, and ThD, produced from thorium. In the case of certain pure uranium minerals which are nearly or quite free from thorium, the lead contained in _these minerals is to be expected to consist only of RaG and AcD, unless the mineral initially contained common lead. Many uranium minerals contain appreciable proportions of

- thorium and their lead will therefore contain ThD in * An address delivered before the Northeastern Section upon addition. Up to the present no thorium mineral has

receiving the Theodore.Wil1ia.m Richards Medal, April 13, 1934. and reprinted from .The Nudcus of May, 1934, by heen discovered which is entirely free from uranium, Dr. Avery A. Ashdown. although in the case of the mineral thorite the percent-

age of uranium is low. Lead contained in thorium minerals will therefore consist largely of ThD with smaller proportions of RaG and AcD, together with common lead if the mineral initially contained common lead. As originally suggested by Boltwood, if the rates of production of RaG and AcD from uranium and of ThD from thorium are known the time which has elapsed since the mineral crystallized may he computed from the percentages of lead, uranium, and thorium contained in the mineral a t the present time. The approximate formula employed for determining the age of a mineral is :

Age = % Pb % U + 0.36% Th X 7600 million years,

in which the factor 0.36 represents the relative rates of production of lead by uranium and thorium. This factor is still somewhat uncertain, and the above for- mula makes no allowance for the fact that a t the present time the quantities of uranium and thorium are less than they were initially-for very old uranium minerals only eighty per cent. of the original; that is, twenty per cent. of the uranium has been convertedinto lead. Correction for the latter circumstance reduces the computed ages. This method of age determination, which is suoerior to that of findine the helium content - with minerals rich in uranium and thorium on account of loss of helium by the mineral in the past, has given a much more enlarged idea of the age of rocks than Wat held earlier.

However, the method is not free from difficulties. If the mineral after its formation has by a natural process, such as selective leaching, undergone altera- tion in composition the lead ratio loses in significance, and while signs of alteration are sometimes obvious, ab- sence of these signs is not conclusive proof of absence of alteration. Furthermore, the presence of common lead is always to be feared. With the object of obtaining evidence as to the nature of the leads found in radio- active minerals, the atomic weights of many specimens of radiogenic leads have been determine+ The first paper on this subject was presented to the Journal of the American Chemiml Society by Professor Richards with Max Lembert in May, 1914, and this was closely followed by one by Honigschmid and Horowitz. Since then many determinations have been made, with results covering the range 205.94 to 207.90, that is, a spread of two atomic weight units, while that of common lead, irrespective of its age or origin, seems to be very close to 207.21.

An exact knowledge of the masses of the lead isotopes and of their proportions in mixtures is desirable for several reasons. In radioactive series, from the masses of the parent isotopes and their products, the masspe-

fects which accompany the emission of a and P particles may he compared with the observed energy emission which accompanies these changes; or, from a knowl- edge of the energy changes together with the masses of the products, the masses of the parents may be found. As it happens, the masses of the parents are a t present somewhat uncertain-that is, those of uranium isotapes, radium, and thorium. In the second place the propor- tions of the isotopes of lead found in uranium and thorium minerals should throw light on the relative speed with which disintegration series of different isotopes proceed. This also is a matter of some un- certainty. Without this knowledge it is difficult to gage the quality of the lead found in radioactive minerals and its suitability for age determination of these minerals.

I should now like to show you the results of recent mass-spectroscopic examination by Aston of both com- mon lead and certain radiogenic leads. Aston finds that referred to O l e = 16.0000 the masses of the individ- ual lead isotopes are slightly larger than integral. about 206.01, 207.01, 208.01, etc. Converted to the chemical basis by the factor 1.00022 these become 205.96, 206.96, 207.96, etc. (See table below.) You will notice that with common lead the chief isotopes are 206, 207, and 208 and that with the five radio- genic leads no other isotopes appear to be present. In the case of the Katanga and Morogoro samples, which were found by chemical analysis to be free from thorium, the isotope PbZo8 seems to be absent. This is in accord with the prevailing opinion that this isotope is the product of the decay of thorium. In these two samples the ratio of PbZo8 to PbZo7 is very nearly the same, as might be expected if the& two isotopes are the products of decay of two uranium isotopes. The two minerals happen to be of about the same age, 600 million years, so that there is here reason to believe that in uranium of this age the decay products Pb2" and PbZo7 will be found in the average proportion 93.2 to 6.8 or 13.7 to 1.

One of the questions still to be decided is whether the uranium isotopes decay a t the same rate, a common opinion a t the present time being that the uranium isotope which is the parent of PbZO' decays a t the greater rate. If this is so, younger uranium would always contain less of this isotope-of uranium, and lead in a younger mineral would therefore contain less of the product. PbZo7 referred to PhZoe and would therefore possess a lower average atomic weight than that from an older mineral.

But certain facts appear to me to be opposed to this idea. It seems not unreasonable to assume that in the case of uranium minerals free from thorium and of the same age, the purest uranium lead will be that of lowest

atomic weight since the presence of common lead will invariably raise the observed value. Recently Mr. Alter in my laboratory examined an interesting sample of lead from a clean specimen of Katanga pitchblende. This is a hlack mineral hut is permeated with microscopic veins of yellow material which could he separated from the hlack with hydrochloric acid. Lead from the extract was found to have the atomic weight 205.97, while that in the insoluble black.materia1 gave the atomic weizht 206.00. The cause of the difference is far from sear, but both values are lower than any others which have been fonnd for Katanga material.

common lead is distributed in accordance with the proportions found by Aston the isotopic ratio of PbZoB to PhZo7 becomes 88.5 to 7.0, or 12.7 to 1, instead of the original 11.3 to 1. This ratio is not far from that found for the African samples, 13.7 to 1. It is worth adding that there is also mineralogical evidence ihat this pitchhlende is contaminated with common lead.

In the following tahle the above corrected values for uranium lead are arranged in the order of the geologic age of the minerals.

Approximate Age Atomic Weight Million Year. of Uranium Lend

~ ~ .~~ ~ ~ ~ ~ - Kolm Not long ago Mr. Bliss examined the ash of a shale- natmp. ~tehblende 400 205.9&206.01 600 205.97-206.02

like material called kolrn found in Sweden. This ash ~~~2Z"E~~$$ . . , e lnnn 600 206.02 ?",I 07 . . .. .. -. ----... ---" contains uranium hut apparently no thorium. Its lead gz! z2 f~;~~g$hlade 1500 1500 208.05 205.90 has the atomic weight 206.01. Since the mineral ash from kolm contains only 0.075 per cent. of lead it may I t i s difficult to see in tliese figures any defjnite trend verv well be that the small ~ r o ~ o r t i o n of lead univer- in the relation of the atomic weizht of uranium lead s& distributed in the eartks .crust was sufficient to to the period during which the radyogenic lead has been affect the atomic weight of the uranium lead in kolm. forming. In other words, there is no evidence here that The correction for this would he -0.01 to -0.03 unit, the rates of decay of uranium isotopes differ by a con- according to the proportion of lead assumed for rocks, siderable amount, although atomic weights of radio- 0.00075 to 0.0022, making the atomic weight of uranium genic leads have been used as an argument to support lead in kolm from 205.98 to 206.00. The age of the the view that the decay of the uranium parent of PhZo7 kolm is probably around 400 million years. is more rapid than that of the parent of PhZo6.

Broggerite from Moos, Norway, is somewhat older Since the higher values in the tahle'may have been than the Katanga minerals, about 1000 million years. dected by the presence of common lead, it seems Honigschmid found the atomic weight of lead from reasonable to assume that the lower values more one specimen to he 206.063. This sample, however, nearly represent the lead produced from uranium. contained thorium, 4.36 per cent., as well as uranium, From the tahle a conservative estimate of the atomic 67.3 per cent. If one assumes fu-st that thorium pro- weight of uranium lead seems to he 206.00, and if the duces lead 0.36 times as fast as uranium and second that ratio of PhZo6 to PbZo7 is 13.7 to I, as found by Aston thorium produces only PhZ07.96 it is possible to calculate for Katanga and Morogoro leads, the isotopic weights the atomic weight of the uranium lead remaining to he of these isotopes must he 205.93 aird 206.93, values ap- 206.02, a value like that found from some of the Ka- preciably lower than Aston's, 205.96 and 206.96. tanga samples. On the other hand, from radioactive measurements

I now come to two of the oldest known minerals of the present rate of production of AcD seems to he no fairly authentic age; one, from the Black Hills of South greater than 4 per cent. of that of RaG. If this has Dakota, is a pitchhlende of which the age is supposed been nearly constant through the agFs the ratio of RaG t o be nearly 1500 million years. rn one specimen the to AcD should he around 25 to 1, and if uranium lead percentages of uranium and thorium weqe 66.9 and 2.0, possesses the average atomic weight 206.00 the isotopic -respectively and the atomic weight of lead extracted weights of RaG and AcD would be 205.96 and 206.96, from it was found by Richards and Hall to be 206.07. values identical with Aston's. Against this must be If allowance is made for thorium lead in the fashion placed Aston's determination of the ratio of RaG and which I have just described the atomic weight of AcD in lead from Katanga and Morogoro, 13.7 to 1 as -uranium lead is found to be 206.05. stated before. -

The other, pitchblende from the newly discovered The results with materials from certain other miner- deposit near Great Bear Lake, Canada, is of approxi- als are puzzlmg. Lead from uraninite from Wilher- mately the same age, 1500 million years. Dr. Marble force, Canada, has the atomic weight 206.20. This has found the atomic weight of its lead to be 206.05. mineral contains 53.5 per cent. uranium and 10.4 per Although the mineral is nearly if not quite free from cent. thorium. Aston finds the ratio Pb206:Pb207:Pb208 thorium, Aston has found the lead to contaim not only to be 85.9:8.3:5.8, with no other isotopes. From the Ph206 and PhZo7 hut also PbZo8 in the proportion 89.8: percentages of uranium and thorium, the per cent. of 7.9: 2.3. In the absence of thorium, one can only at- thorium lead should he 6.4, a value slightly highq than tribute the PhZo8 to common lead; hut since this Aston's 5.8. The ratio of Ph2" to PbZo7 is 10.4:1, a isotope is only half of common lead, the percentage of ratio again higher than Aston's although the age of this common lead must he twice as large, 4.6. If allowance mineral (1000 million years) is not very much greater i s made for this percentage of common lead, of atomic than that of the Katanga specimens; that is, the weight 207.21, th6 atomic weight of the residual Wilberforce lead contains 2 per cent. more PhZQ7 than uranium lead is 205.99. Further if 4.6 per cent. of corresponds to the ratio 13.7:l. To provide for the

excess of PbZo7 ten per cent. of common lead would be that diierent experimenters obtain different results required, corresponding to 5 per cent. of PbZo8, and with the same material. On the other hand, the com- leaving little PbZo8 to correspond to the thorium. position of the lead chloride used for analysis may not be

Thorite from Moos, Norway, contains lead of atomic normal, although attempts to cause it to vary in com- weight 207.90. From the percentages of thorium, position have heretofore been unsuccessful. 30.1, and uranium, 0.45, the percentage of uranium It seems equally unlikely that the mass values and lead should be 4.0. Aston, however, finds PbZ0" proportions of isotopes found by the physical method Pb207:Pb208=4.6:1.3:94.1; SO that if all Pbzoe and are seriously in error since atomic weights calculated PbZO' is derived from uranium, the percentage of from these values agree surprisingly well with those uranium lead is 5.9. Furthermore the ratio of PbZoB to found by chemical methods. Pb207 is high, 3.5:l. If the difference between the A possible explanation of the discrepancies is that percentages of uranium lead calculated from the per uranium and thorium leads may be more complex than cent. of uranium, 4.0, and found by Aston 5.9, is as- is a t present believed. In the case of thorium, for cribed to common lead, Aston's ratio becomes instance, the atomic weight, 232.12, is so far above an 4.1:0.9:93.2, the ratio of PbZoB to PbZ0' still being too integral value that one suspects the existence of an high. isotope higher than 232 which might be the parent of

How much can be explained on the basis of selective a lead isotope of mass higher than 208, while Aston's leaching of minerals by natural processes is difficult to analysis of lead from both thorite and Wilberforce say. It is generally believed that uranium is removed uraninite shows a proportion of PbZoT which is hard to from a mineral by leaching faster than lead, and lead explain. faster than thorium. This might account for a dis- Until more information of all kinds has been secured, crepancy between the proportions of uranium lead and it seems unlikely that it will be possible to fit together thorium lead found by the mass spectrogaph and those perfectly all the pieces of this picture puzzle. One of calculated from the percentages of uranium and the restrictions lies in the scarcity of suitable uranium thorium. On the other hand, all the isotopes of lead and thorium minerals. Few uranium minerals have yet would be removed a t rates proportional to their per- been discovered which are certainly free from both centages at the time, and no change in the relative thorium and common lead, while the purest thorium amounts of RaG and AcD could be produced in this minerals known contain appreciable proportions of way. uranium. Perhaps the explanation of the difficulty

The whole situation seems to me far from clear. So will come from an entirely different direction. At any far as the atomic weights are concerned the conven- rate this subject provides a problem in which geologists, tional analysis of chloride used for their determination physicists, and chemists may collaborate with cousider- presents no serious difficulties, and there is no evidence able interest.