T H E ITAILE?;CEOF T H E -4RGOS GROCP -AS DETERM I S E D FROM T H E JIOLECLLAR COHESION BY .\I,BERT P. M.lTHE\T-S
The valence of the argon group of elements is one of the most interesting problems in chemistry. They are very generally regarded as zero valent. chiefly owing to the position they take in the periodic system between strongly electropositive and electro-negative, univalent elements. That they are monatomic is undoubted, b u t they might he monatomic, like mercury vapor, and still have \-alence. Ramsay' made the suggestion, indeed, that they combine into molecules a t other than ordinary temperatures. To account for the atomic iveight of argon, which computed from the density is 39.9 if the gas is monatomic, he suggested that argon is a mixture of many monatomic molecules with a fen- diatomic molecules. The ratios of the specific heats as determined is 1 , 6 5 9 ; \Thereas if there xvere j percent of diatomic molecules it \vould be 1.6;s. The theoretical number, if the gas is entirely monatomic. is I ,667. -After discussing- this possibility, hen-ever, Ranisay says: Eut on the J\-liole the presuniption is against the hypothesis that argon is a mixture of monatomic and diatomic niolecules." There is some e\-idence that argon is n o t entirely lacking in chemical afinity. Eerthelot,' by the action of the electric discharge on a mixture of argon a n d benze'ne s-apor, or of a r ~ o nand carbon bisulphide, produced a b r o ~ r i i s hdeposit on the glass from ivhich argon could be reobtained. Ramsay," in commenting or1 the absence of the argon lines in the sun's spectrum. suggests, as a reason, that it enters into combination only a t high temperatures, these compounds being endothermic; ant1 he cites' several observations indicating "
'
Ramsay: "Gases of thc .4tnioEphere," I.clnduri, 1). 231 [ i r p . ? ) . Herthelot: Comptei rendus, 1 2 0 , 581, 060, r 3 r t (18(),j);124,I 13 , 18cj:). R a m s a y : Loc. cit., 11. 2 6 1 . * RamsaJ-: Sce footnote, p. 5 3 8 , tci articlc b>- C . Trcntiin Cookc. Zcit. phys. Chem., 5 5 , 53; (1906).
338
Albert P . Mathews
a union of argon with zinc, mercury and some other elements. Thus in a Plucker tube the cathode metal disintegrates more rapidly when argon under low pressure is in the tube than when nitrogen is there, and Ramsay interprets this to mean that a volatile compound is formed under the influence of the intense energy a t the surface of the electrode and this compound dissociates again setting free the metal, which deposits on the glass. Under his direction C. Trenton Cooke’ measured the vapor tension of zinc, cadmium, sulfur, mercury and some other metals a t high temperatures in the presence of various gases and concluded that the tension of zinc in argon was 1 2 percent above its tension in nitrogen. Cadmium behaved similarly in helium. Helium seems to be in some kind of a union in fergusonite, and to be capable of feebly uniting with platinum. I t may be recalled, also, that the solubility of argon in Ivater is greater than that of helium and nitrogen; and this may be urged as indicating some kind of affinity between water and argon. Chemically, then, these gases, though inert. are not entirely indifferent. From their action on light,’ also, a certain argument may be made for their possessing valence. Thus, according t o Drude, the dispersion of light in the blue end of the spectrum is due to the valence electrons, and in the red end to the vibrations of the electrically charged atomic groups. If only the valence electrons affect blue light, these gases must also have valence electrons, since they refract light like other gases. The question whether these elements have valence, or not can be put to a decisive test through their molecular cohesion^.^ There is no question t h a t they possess cohesion, since they can all be liquefied. They behave like all other gases in this respect. Molecular cohesion in all other substances examined is a function of the product of the molecular weight b y the number of valences. I t has been shown for a
204,
C. Trenton Cooke: LOC.cit , p. j 3 7 . Cuthbertson, C., “Refractive Indices of the Elements,” Phil. Trans., 323 (1904). Nathews: Jour. Phys. Chem., 17, 181 (1913).
The I'aleme o j the -4r-gotL Gr-oup, Etc.
339
great number of substances that b I X , a factor proportional t o "a" of van der I17aals' equation, is equal, n h e n expressed in dynes, to 2.98 X (mol. n-t. X S o . of valences)-1 substance having cohesion cannot, therefore, have zero valence. If i t had no valence it could have only gravitational attraction between its monatomic molecules, no cohesional attraction These gases have cohesion and they must, therefore, have x-alence. Their valence, 1 1 , can be calculated from their critical data by the formula: f r ) IZ = o 0043 T,' P,'2 imol. u-t 1 ; or, I I =- a i ' x 3 2 x IO' (mol. u t . ) ; or ( 2 ) I I = 1 . 2 1 x IO-' [ITL'Tc)'imol n-t.). The first formula, nhich is derived from the ordinary formula for computing " ( 1 " from the critical temperature and pressure, qives values for " ( i , and hence for 1 2 , a little loner than the second formula in the case of simple gases. The second formula computes ' ' a ' from the surface tension, as shown in my former paper In Table I , I have given both values and I regard the second A S the more correct; b u t as the critical density of n o t all the qases is known, I ha\-e had to rely on the first formula for a comparison. The results given b y the two formulas are not \ridely different The valence, i i , together with the critical data' used in the calculation are given in Table I 1 .
"
T ~ B L E : I-COJIKT PER
Substanccs
ITIOS
OF THE .IYER.\CE SVNBER OF \ ~ . i L ~ ; ~ ~ I ; ~ THE ~IOLECUI,.IR COHESIUS
~ J O L E C U L EFROV
Xurnbcr of Yumbcr of for- 1 alences by mula I formula 2
d,
1''
T, (.lbs ~
Helium Helium Seon -1rgon Krypton Xenon
j
5'
s o
61 I Ijo j6 2 1 0 53 269 6
2 2
7j 75
o
29 4b
54 3 jS 2
o 04 o 12
OGj
o 065 -.
0
j09 ~
07
0
I2
0 32
-
I
0
j
j
I
I2
I
23 80
I
-
1
3.5
1
95
I n my preliminary paper, Science, S S , 36, 6 (1912), in Table I a mistake occurred in the computation of hcliurn The ialue z 90 is nrong. The critical density n a s taken as o 01j instead of o 065
Before discussing these results a word may be said about the reliability of the critical data. Those of argon and xenon are perhaps the most certain; krypton, neon and helium follo\i- in the order named, helium being least certain. Onnes' gives j . j oabsolute as the critical temperature of helium, b u t as this makes helium quite aberrant in several particulars,' I have also computed the s-alence assuming the critical temperature t o be X o as suggested by Den-ar.3 The critical data of neon are somewhat uncertain, due in part to the very loncritical temperature and in part to the great difficulty in separating the gas completely from helium. -1 little impurity of the latter gas n-ould have the effect of making P, too high. It is clear from the table that all of these gases possess valence. They are not zero \-alent as they are supposed b y many to be. Furthermore, the average number of valences per molecule is in no case an exact integer, although in argon and xenon it is not far from a whole number. Since these gases ha\-e their critical data most accurately determined I z i t first supposed, as I published in my preliminary paper, t h a t argon \\-as Lini\-alent. b u t slightly associated into diatomic molecules, thus briIiging the aL-erage number of valences per molecule a little high; and that xenon was di\-dent. I attributed the del-iation of the other gases from univalency, t o the inaccuracy of the d a t a . .I careful examination of all the facts, however, has led me to abandon this explanation for Ivliat seems to me to be a better one, since it explains all the facts. I n the first place, I have not been willing to abandon the idea that valences are indivisible. If \ye assume, as Lodge' suggests, t h a t some of the lines of force from each valence attach themselves to several atoms, or even lvancler outside .
~~
I
2
I
Onncs: Proc. .lmsterdam Xcad. Sci., 1 3 , I 100 I I C ) I I ) . Rankin: "On a Relation bctn.ccn \.iscosily and Atomic U-cight of Phil. Mag., [ h ] 2 1 , q ~ j(191I ) . I k w a r : Xrticlc "Liquid Gascs," Ilncyclopacdia Uritannica, 16, :qc),
IIthed. Lodge. Katurc. 70, r;G
(~gcq),
The I*nleiice o j the Argo?? Group, Etc.
341
the molecule, and thus split the valence u p ; or t h a t there are partial valences in the sense of Kaufimann,' it seems to me that n-e might as well abandon the whole valence idea. I t entirely loses its usefulness. ,Again our structural formulas become so indefinite, if the valences are regarded as split up, as to be nearly useless. The explanation which I sought \vas one irhich n-ould explain why lye appear to have fractional valences in this case, b u t really d o not have them. That my first explanation was wrong, was indicated b y the uniformity with which the valence increases from helium to senon. The deviation, too. from unii-alency in the case of neon is so great that it lies outside the limits of error. The critical pressure Ivould need to be 1.4atmospheres imtead of that recorded of :?g atmospheres, in order that neon should ha\-e one valence to each molecule. The uncertainty of the critical pressure is far less than this. I belie(-e the reason that the a l - e r q e iiuniber of \.alences is a fraction in these gases is as follon-s: -111 of them are i n reality zero \ - d e n t . so i'as a r their chic! valences are concerned, 1iut like many. if not all, other elements, they have the poIi-er of openinq u p tlro residual 1-aleiictrs. Ey their residual valences, tliereiore, they are all bivalent. These t\vo \-alences. like most. or all, other residual \-aleiices, are of opposite electrical siyn, one being positi\-e. the other iieqati\-e. S o t all thc atoms hL1i-e these residual \.alences open :it the sanie iiihtaiits but aln-aj-s some of the atoms have then; closed. The n~oleciilarcohesion. as I ha\-e already poiiitecl o t t t , is not influenced by these valences \Then they are in :.I closed or rcser\-e state, or, Tve might say, iritliclrairii within the a t m i ; it is otily aCected by the valences actually extending betn-cen atoms, or ope11 and in :I state in which they may combine. Iri xenon nearly all the atoms, or a t any rate, c;o percent of them, have the r-alences open, and the average i.alence per atom, or molecule, is, therefore, I .SO--I.gj; in krjrpton about 6>j percent of the atom ha\-e open \-alelices, and 3 j percent are clo:;ed, so that the nL-erage valence is about 1.3~1: i n argon,
about 60 percent are open and 40 percent are closed, the average valence being about 1 . 2 0 ; in neon, 16 percent are open and 84 percent are closed, the average valence being about 0.32; and in helium only about j percent of the valences are open, 95 percent being closed, giving an average valence of 0.10.This explains, then, n-hy the elements appear zero valent in the periodic table, since they are zero valenl as far as their chief valences are concerned ; why, nevertheless, they appear to have some weak chemical affinity, and cohesion; why they refract and disperse light; and also why the average valence is fractional rather than being a whole number. It also explains more than this. I t enables us to understand the easy dissociation of the molecules into atoms. Unlike atoms that are bound together into molecules b y their chief valences, no electrical stresses are set up in the argon elements when dissociation into atoms occurs, because each atom having a positive and a negative valence becomes a t once electrically neutral. I t is Ire11 known t h a t compounds formed from residual 1-alences partake of the nature of molecular compounds and break up very easily. Neither their union, nor their dissociation, involves much, if any, energy exchange. Such compounds are often called, indeed, molecular compounds. X double bond of this kind is always a weak bond in any molecule, which easily breaks n-here the double bond is. n’ere they univalent their dissociation into atoms n-ould be very hard t o understand. I suppose we may picture the opening of these residual valences in the manner suggested by Sir J. J. Thomson, as being due to a rearrangement of the electrons within the atom so t h a t an excess of negative electricity is temporarily produced in one spot, and of positive at another spot on the surface of the atom. These excesses are the valences. I n closing, i t is not without interest t o compare the valence numbers computed above from the cohesion, with the refractivity as determined by Cuthbertson. Their refractivities are in the proportion I , 2 , 8,1 2 and 19. Cuthbertson,
C. Phil. T r a n s ,
204, jzj ( 1 9 0 j ) .
The I'alence
Helium Seon
'
0)
36 3 68 j
the Argon Group, Etc.
313
I
0 1
I
I 9 7 82
032
3 2
Argon I 12 I 2 84 Krypton j 425 , 11 7 I 2 3 0 ) 689 18 98 I 80 Xenon There is a general similarity, b u t not an identity.
I1 2 12
3
18
0
The principal facts and conclusions of the paper are: The molecular cohesion, confirming other properties, shon s that the argon group of elements have valence A computation of the average number of valences per molecule from the molecular cohesion gave the folloning results : He, 0.1; S e , 0 . 3 2 ; Air, 1.12; Kr, I 23; Xe, 1.80. The valences are apparently fractional, and not whole numbers. The conclusion is t h a t these elements are all zero valent, as far as their chief \-alences go, b u t each is divalent in its residual valences. At any one instant of time only a certain proportion of the atoms, varying in the different gases, ha\-e their residual valences open, consequently the average number of valences actually open, or active, per molecule is less than two One residual valence is positive, the other negative; and hence the combining power of the atoms is very weak, since on dissociation an electrically neutral atom is formed, b y the saturation n ithin the atom of the oppositely charped valences This explanation enables us to understand, also, nhy there is a progressive increase in solubility of these gases in water from helium to xenon if solution be a process involving the union of solvent and solute through their residual valences 1 izt;oszty 01.ClztLago
