BING-MANFUNG
596
The Electronegativity of Noble Gases
by Bing-Man Fung Contribution N o . 9140 f r o m the Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California (Received September 29, 1964)
The electronegativity of xenon is evaluated from the Xe-F bond energy applying Pauling’s new concept of the “transargononic bond.” Values of electronegativity of other noble gases are calculated from their estimated covalent radii as well as by the Iczkowski-Margrave forniula connecting the electronegativity with ionization potentials. The selected values of the electronegativity are 2.5-3.0, 4.4, 3.5, 3.0, 2.6, and 2.3-2.5 for helium, neon, argon, krypton, xenon, and radon, respectively. On the basis of the estimated electronegativity of neon, new noble gas compounds such as “e+, CF3Ne+,and BF3Ne are predicted.
The concept of electronegativity was introduced by Pauling’ and has been used extensively in relation to the chemical and physical properties of elements and It is described as “the power of an atom in a molecule to attract electrons to itself.”l The values of electronegativity have been calculated for alniost all elements in the periodic table by several different methods whose results are in good agreement with Pauling’s values3-’; the electronegativity for noble gases was estimated very crudely with XIulliken’s foriin~la,~ taking the electron affinity of the noble gases to be However, in view of the growing interest in noble gas conipound,’O it is more desirable to investigate in greater detail other possible ways of calculating the electronegativity of noble gases in order to get better insight into their che~nistry. We shall discuss three methods and then list the results of numerical calculations. Bond Energy. Pauling1,2 defined the electronegativity .z by
D(X-B)
2
[D(A-h) X D(B-B)]”’
+ ~ O ( X A- XB)’ (14
or
D(A-B)
=
‘/,[D(A-A)
+ D(B-B)] + 2 3 ( ~ - ~XB)’
(lb)
where D(.A-B) stands for the bond energy between two atoms A and H in kcal./niole, etc. These foriiiulas can be applied only to nornial covalent bonds, in which each The Journal of Physical Chemistry
atom has the electronic configuration of noble gases For compounds (e.g., PC16,Pz06,etc.) to which valence bond structures of another kind are assigned, Pauling introduced the term “transargononic struct,ure” to describe the electronic configurations beyond those of noble gases.“ For example, t’he bond energy of the two “transargononic bonds” in PC16 is assigned t’he value of 40.1 kcal./mole froni the reaction PC13(g) 2Cl(g) -+ PC16(g)4-2 X 40.1 kcal./niole. In Table I we list value^^^-^* of transargononic bond energy for several other types of bonds containing halogen atoms. The assignment of transargononic
+
(1) L. Pauling, J . Am. Chem. Soc., 54, 3570 (1932). (2) L. Pauling, “The Nature of t h e Chemical Bond,” 3rd Cornell University Press, Ithaca, N. Y., 1960, p. 88.
Ed.
(3) R. S. Mulliken, J . Chem. Phys., 2, 782 (1934); 3, 573 (1935). (4) W. Gordy, Phys. R m . , 69, 604 (1946). (5) W. Gordy, J . Chem. Phys., 14, 305 (1946). (6) W. Gordy and W. 3. 0. Thomas, ibid., 24, 439 (1956). (7) R. P. Iczkowski and J. L. Margrave. J . Am. Chem. Soc., 83, 3547 (1961). (8) R. E. Rundle, ibid., 8 5 , 113 (1963). (9) A. B. Neiding, Russ. Chem. Rev., 32, 224 (1963). (10) H. H. Hyman. Ed., “Noble-Gas Compounds,” University of Chicago Press, Chicago. Ill., 1963. (11) L. Pauling in “The Law of Mass-Action, a Centenary Volume,” Det Norske Videnskaps-Akademi i Oslo, Univesitetsforlaget. Oslo, 1964. (12) F. D. Rossini, et al., Ed., National Bureau of Standards Circular 500, U. S.Government Printing Office, Washington, D. C.. 1952. (13) W. H . Evans, T. R. .Munson, and D . D. Wagman, J . Res. .Vafl. B u r . Std., 5 5 , 147 (1955). (14) R. K. Steunenberg, R. C . Vogel, and J. Fischer, J . Am. Chem. SOC.,79, 1320 (1957).
ELECTRONEGATIVITY OF SOBLE GASES
597
bonds in a compound is somewhat arbitrary"; molecular symmetry and other factors of convenience are considered; e . g . , in Table I the reactions S 6F + SF6, etc., rather than SFZ 4F + SF6, etc., are listed because of the equivalence of the six S-F bonds in the compound SFO.
+
+
Table I : Some Values of Transargononic Bond Energy of Bonds {ContainingHalogen Atoms"
Bond
P-c1 P-Br Sb-CI S-F Se-F Te-F C1-F Br-F
I-F a
Transargononic bond energy, Reactions considered (all in kcal./ gaseous state at 298O K.h mole
+ + +
Pc13 2c1 Pc1, PBr3 2Br PBrS SbC13 2C1- SbClb S 6F SFs Se 6F SeF6 Te 6F TeF6 ClF 2F -+ ClF3 BrF 2F BrF3 BrF 4F BrF, IF 4F IF5 IF 6F + IF7
+ + + + + + + +
-+
-+
-+
-+
-+
-+
-+
-+
40 35 39 71 67 79 32 50 45 62 53
Difference between colNormal bond umna 4 energy, kcd./mole and 3, Calcd. kcal./ from (lb) Exptl. mole
73 60 72 96 99 118 70 74
76.3 64 74.3
61 60
88
67
.. .
.
..
33 25 33 25 32 39 38 24 29 26 35
yet demonstrated the existence of noble gas molecules. The interaction between two noble gas atoms would not exceed the van der Waals interaction,I8 which has a value of about 0.5 kcal./mole for noble gases. Therefore, the vaguely defined noble gas diatomic molecule~'~,'* cannot be regarded as containing real trahlsargononic bonds, and, consequently, for noble gases the term D(A-A) in ( 2 ) can be dropped compared with the normal covalent bond energy D ( X - X ) of halogens. Thus, we can calculate the electronegativity of a noble gas from the bond energy of its halides by
D(A-X)
x
=
+ 30 = '/z[D(A-A) + D(X-X)] + WXA XX)'
+ 0.50
0.31(n+)
(3)
(4)
where n is the number of electrons in its valence shell and r is the covalent radius. Equation 4 can be applied to estimate the electronegativity of the noble gases if their covalent radii are known. Ionization Potential and Electron Afinity. Nulliken's definition for the electronegativity of an element is given by3
x=- I1
As can be seen from Table I, the normal covalent bond (energiescalculated by Pauling's formula (lb) and the transargononic bond energies differ by a certain aniouiit, which lies within 24 to 40 kcal./niole for bonds containing halogen atoms. If the value 30 kcal./niole is used as a correction term in (lb) for those bonds, the result will be quite satisfactory for the estimation of electronegativity, which is less sensitive to small variations 111 values of bond energy. Therefore, we obtain
2 3 ( 2 A - XX)'
Covalent Radii. Gordy4 proposed that the electronegativity of an element can be related to its effective nuclear charge and covalent radius by
Data taken from ref. 12-14.
D(A-X)
+ 30 = '/zD(X-X) +
+ I-1 2
(5)
where I l is the first ionization potential and I-1 the electron affinity of the element. Expressing these quantities in e.v. and bringing them to Pauling's scale, one obtainsIg
The ionization potentials of most elements including the noble gases are tabulated,'o but the electron affinity values are known experimentally for only a few elements. Theoretical calculations of electron affinity have been proposed by several 1 9 23-26
(2)
where A-X denotes a transargononic bond containing a halogen atom. Since the correction term 30 is only approximate, eq. 2 should not be applied to elements with values of electronegativity too close together or too far apart. I n evaluating the electronegativity of the noble gases we must also know the bond energy between two noble gas atoms. Though some noble gas diatomic cations'.! 16 and certain interactions in a diatomic xenon system17 were reported, no experimental evidence has
(15) J. A. Hornbeck and J. P. Molnar, Phys. Rezb.. 84, 621 (1962). (16) H. T. Davis, S. A . Rice, and L. Meyer, J . Chem. Phys.. 37, 947 (1962). (17) H. C. Torrey, P h y s . Rev., 130, 2306 (1963). (18) N. Bernardes and H. Primakoff, J . Chem. Phys., 30, 691 (1959). (19) H. 0. Pritchard, Chem. Reo., 52, 529 (1953). (20) C. E. Moore, National Bureau of Standards Circular 467, U. S. Government Printing Office, Washington, D. C . (21) G Gloker, Phys. Rev., 46, 111 (1934). (22) D. R. Bates, Proc. Roy. Irish Acad., 51, 151 (1947). (23) H. A. Skinner and H . 0. Pritchard. Trans. Faraday Soc., 49, 1254 (1953).
Volume 69, Number d
February 196d
BING-MAXFUNG
598
Iczkowski and lIargrave7 suggested that the total energy of electrons in an atom or ion with a net chaxge ( - N ) can be represented by
E’(N) = a N
+ b N Z + cN3 + d N 4
(7)
and t,he electronegativity expressed by
assuming the E-N curve to be continuous a t N = 0. From (7) ~ t get l the electronegativity by Afulliken (eq. 5) x = -a - c (9) which differs from (8) by -e. The coefficients a,b, c, and d can be obtained by fitting experimental values of the ionization potentials (and electron affinity where available) into (7) and solving the resulting simultaneous equations; the results show that c is always much smaller than a and b,’ hence, expressions 8 and 9 are not very different. Therefore, we can calculate the electronegativity of the noble gases without knowing their electron affinity.
Results of Calculation Bond Energy. Detailed therniodynamic data are
by S a n d e r s o ~ i . ~ Starting ~ from this, we can deduce the covalent radii of other noble gases. Figure 1 shows the trends of univalent radii2 and covalent radii33of atoms in the last four groups of the periodic table. Since the univalent radii of the noble gases have practically the same trend as those of the other groups, we may hope that their covalent radii behave in the same way. In Figure 1 the dotted line, which is drawn from the starting experiniental value r X e = 1.30 -I., gives the following covalent radii of the noble gases (in units of i.): Rn, 1.40-150; Xe, 1.30; Kr, 1.09; Ar, 0.94; Ne, 0.70; He, 0.40-0.60. I n estimating these values, Schomaker-Stevenson values for the covalent radii of the first-row are used rather than those by Pauling2 because we are going to apply (4) in which the values given by the former authors are to be used.4 For the sake of comparison, we list the values by G i l l e ~ p i e( ~r X~e = 1.30 A., TKr = 1.11 .“I.,and TAr = 0.95 A.) which are quite close to ours. From the values of covalent radii estimated in the above manner, we have deduced, as follows, the electronegativity of the noble gases according to (4): Rn, 2.3-2.5; Xe, 2 . 7 ; Kr, 3.0; Ar, 3.5: Ne, 4.5; He, 2.12.8. Ionization Potential. From the values of the ionization potentials of the noble gases20we can calculate their electronegativity according to (7). The results of -2a/5.5, obtained from (8) after being brought to
available for xenon tetrafluoride and xenon hexafluoride.l0 The average bond energy taken from those works is 31.0 kcal./mole. Substituting this into (3) we get xxe = 2.6. If the correction term is allowed to vary from 2 1 to 40 kcal./mole, the value of x x 0 varies Table 11: Values of Electronegativity of Noble Gases from 2.7 to 2.Ei, which is well within the error of such an Calculated by the Iczkowski-Margrave Formula approximat ion. Though some krypton and radon compounds have Two terms Three terms Four term8 been reportcld,’O2 7 - 2 9 no detailed data about their He 3.5 .. ... properties are available, obviously owing to the inNe 4.3 4.7 4.1 stability of the krypton compounds and the difficulty of Ar 3.6 3.7 3.4 Kr 3.2 3.4 ... handling thc highly radioactive radon compounds. Xe 2.8 3.0 ... Bartlett30 estimated the bond energy of krypton tetraRn ... ... ... fluoride to be -18 kcal.,’niole; granted that this is a reasonable estimation, we have 2~~ = 2.9 froin ( 3 ) . Covalent Radii. The Xe-I; and the Xe-0 bond dis(24) J. L. Margrave. J . Chem. Phys., 22, 636 (1954); 22, 1937 tances in several xenon coinpounds have been meas(1954). ured.lO The average value for the Xe-F bond is 2.00 ‘1. (25) H. 0. Pritchard and H. A. Skinner, ibid., 22, 1936 (1954). in XeF2 and 1.94 in XeF4. In comparing the bond (26) G. Klopman, J . Am. Chem. SOC.,86, 1463 (1964). lengths of halogen fluorides, Gillespie31 indicated that (27) D. K. 1IacKenzie. Science, 141, 1171 (1963). the “long” bonds in the nienibers having a larger nuni(28) A. V. Grosse, A. D. Kirshenbaum, A . G. Streng, and L. V. Streng, ibid., 139, 1047 (1963). ber of fluorinil atoms usually represent the additivity of (29) C . L. Chernick. ibid.. 138, 136 (1962). vovalerit radiio better. For this reason the average (30) N. Bartlett, Endeavour, 23, 3 (1964). v a l w of 1.94 A. for the Xe-F bond in XeF4 is used to (31) R. J. Gillespie, ref. IO, p. 333. calculate the covalent radius of xenon. Faking the (32) R. T. Sanderson, Inorg. Chem., 2, 660 (1963). covalent radius of fluorine to be TF = 0.61 L%.,2 me have (33) V. Schomaker and D. D. Stevenson, J . Am. Chem. SOC..63, 37 ?.Ye = 1.30 *-I. A siniilar value (1.31 K.)was obtained ( 1941). The Joiirnal of Physical Chemistry
ELECTRONEGATIVITY OF XOBLEGASES
t
599
HSb
3*0
2.0n
U Y
Q)
1 .TJ 0
a 1.0-
I ;4 i
0.0 ~
argononic electrons,” Le., electrons in a configuration beyond noble gas structures, would be too large for stable bonding. Therefore, the stability of those conipounds will depend upon the difference between the electronegativity of the central atom and the coordinated atoms, in addition to other factors such as molecular geoinetry. From this argument, we can readily visualize the stability orders of sonie noble gas compounds (radon fluorides > xenon fluorides > krypton fluorides; xenon fluorides > xenon oxides) and the fact that, in photolysis, products of either the F-Xe-Ar systein or the FKr-Ar system contain no argon fluorides while the fluorides of the other two elements are produced.10 Though the above discussion also rules out the possibility of forination of compounds with transargononic structure for the highly electronegative elements, neon and argon, it does not exclude the predicted compound HeF2discussed by Pinientel and Spratley.34 Moreover, we may speculate on some possible neon and argon conipounds not containing transargononic structure. For example, the cation HXe+ is likely to exist owing to the large electronegativity of neon. Coinparing the series NH3,H20, HF, Re, and NH4+,H30+,H3F+,HXe+, we should expect the hydroneonium ion HNe+ to be a very strong acid and neon, an extremely weak base. The hydroneonium ion might not be very stable but should be detectable in the gaseous reaction products of neon and the strongest inorganic acids by spectroscopic and mass spectroscopic methods. If some strong organic acids are soluble in the nonpolar liquid neon, their electric conductivity should be substantially larger than that expected for nondissociated molecules because of the formation of ion pair and the rapid propagation of protons in the form of hydroneonium ion in liquid neon. The hydroargoniuin ion HAr+ may also exist but would be less stable. I n fact, Rank and c o - ~ o r k e r reported s~~ the complexes HCl-Ar and HC1-Xe in their analysis of the spectroscopic data of the gaseous systems of hydrogen chloride-noble gases. Substituted fluorocarbons such as CFJVe+ and the coinpound of neon with boron trifluoride are also possible. Booth and Willson reported36the “conipounds” ArBF3, Ar(BF3)2,etc., from the thermal analysis of the argon-boron trifluoride system; those l ~ c o n ~ p o u n d s ~ ’ are said to be unstable and dissociate above their
,
2
3
4
5
6
Period Figure 1 . Covalent and univalent radii of elements of the last four groups in the periodic table: full points, univalent radii; empty points, covalent radii.
Pauling’s scale, are shown in Table 11. It should be noted here that (7) is, in general, less satisfactory for heavier atonis.’
Discussion Since methods based on different experimental data (for xenon) and theoretical bases (for xenon and other noble gases) have given fair agreement on estiniations of the electronegativity of the noble gases, it is natural for us to express confidence in these results. The selected values of the electronegativity of the noble gases are as follows: He, 2.5-3.0; Ne, 4.4; Ar, 3.5; Iir, 3.0; Xe, 2.6; Rn, 2.3-2.5. Electronegativity is not a nieasurenient of the reactivity of elements; therefore, there is no inconsistency between the large electronegativity values of the noble gases and their cheniical inertness. On the other hand, based upon those values, we can explain certain experimenta,l facts and make some speculations about other possible noble gas compounds. In a compound containing transargononic bonds, those bonds are unlikely to be polarized i n such a way that there is a net negative charge on the central atom; otherwise, the Coulombic repulsion between the “trans-
(34) G. C. Pirnentel and R. D. Sgratley, J . Am. Chem Soc., 8 5 , 826 (1963); Science, 143, 674 (1964). (35) D. H. Rank, 1’. Sitaram, W. A. Clickrnnn, a n d T. A . Wiggins, J . Chem. P h y s . , 39, 2673 (19G3). (36) H. S. Booth and K. S. Willson, J . Am. Chcm. Soc.. 57, 2273, 2280 ( 1 935).
Volitme 69, Number 2
February 1.965
600
W. W. BRANDT AXD R. S. BUDRYS
melting points. The failure to find adducts of argon, krypton, and xenon to boron trifluoride a t 20°K. was reported.'O We feel that neon may form a real compound with BF3, which would have a heat of formation of about -440 kcal./mole. The compounds CF3Ne+ and BF3iSe are expected to be reasonably stable, though not quite comparable to their isoelectronic species CF, and BF,-. The corresponding argon compounds would be at the margin of stability if they can form a t all. Similar compounds of the other noble gases are unlikely.
In conclusion, we are looking forward to niore accurate calculations of the electronegativity of noble gases in the hope of obtaining more valuable informations and predictions with regard to this new and broad field of chemistry, the cheniistry of noble gases.
Acknowledgments. The author is indebted to Professor Linus Pauling for his introduction of the concept of the trarisargononic bond and to Dr. Sunney I. Chan for his many valuable suggestions.
Sorption Rates Indicative of Structural Changes in Solid Polypeptides'
by W. W. Brandt and R. S. Budrys Department of Chemistry, Illinois Institute of Technology, Chicago, Illinois
60616
(Received September 30, 1,964)
The sorption isotherms of HzO on poly-L-leucine (PL) and poly-L-valine (PV) have been measured, as well as sorption rates of H20, CH30H, HCl, and CF3COOH on PL and of H20 and CF3COOH on PV. In certain systems, the rates are found to furnish a sensitive indication of the onset of structural changes in the polymer. Several distinct processes occur in the system H20-PL; they are tentatively identified as the irreversible partial opening of helical segments and the reversible breakage and re-f orniing of intermolecular H bonds. A plot of the sorption isotherms, according to the Frenkel, Halsey, and Hill equation, reveals that structural changes which are too fast to be sorption rate controlling probably occur in certain other sorbate-polypeptide systems. In some of these systems, the apparent diffusion coefficients decrease with increasing sorbate concentrations, presumably owing to the simultaneous effect of these fast structural changes and the nonlinearity of the corresponding sorption isotherms.
Introduction Earlier studies carried out in this laboratory2showed that sorption and desorption of trifluoroacetic acid (TFA) causes certain distinct structural changes in poly-L-valine (IT) and poly-L-leucine (PL), judging from X-ray diffraction patterns taken before and after the sorption experiments. The sorption isotherms obtained for PV shifted in a sequence of consecutive runs, and the rates of sorption increased with sorbate concentration. PL showed distinct but less pronounced The Journal of l'hysieal Chemistry
isotherm shifts. Barrer noted similar shifts and measured concent'ration-dependent diffusion coefficients for various H-bonding sorbates in ethyl cellulose. (1) (a) Abstracted in part from the work of K. S. Budrys to be submitted in partial fulfillment of the research requirement for the 1'h.D. degree in the Department of Chemistry a t Illinois Institute of Technology; (b) this work was supported by Public Health Service Grant A-4324. (2) W. W. Brandt and R. S. Budrys, J . B i d . Chem., 239, 1442 (1964). (3) R. M.Barrer. J. A. Barrie, and J. Slater. J . Polymer Sei., 23,315, 331 (1957).
