James E. Huheey and Caroline L. Huheey I Anomalous Properties of Elements University of Maryland College Park, 20742 I that MOW"Long Periods" of Elements

The phenomenon known as the lantha- difficulties in their separation are well-documented (3). nide contraction has long been appreciated as an impor- Less well-explored are the cases in which the lanthanide tant factor in determining the properties of the lan- contraction results in different properties for the post- thanide and post-lanthanide elements. Appreciation lanthanide elements (4). In general, these may be re- of this phenomenon is of obvious heuristic value with lated to greater effective nuclear charge and attraction regard to research efforts directed towards these ele- for electrons in the heavier elements. ments. In addition there is an implicit pedagogical Table I lists the ground state ionization energies (6) value that has been discussed at some length (1) and for the elements rubidium-xenon (Period Five) and which is included in most descriptive discussions of these cesium-radon (Period Six). The pre-lanthanide ele- elements. It is the purpose of this paper to examine ments cesium, barium, and lanthanum have ionization the physical nature of this phenomenon and extend the energies less than their lighter congeners as expected for concept to other parts of the periodic table. the main group elements. The effect of the addition of fourteen protons to the nucleus and fourteen poorly shielding 4f electrons is a higher ionization energy for hafnium than for zirconium. 811 of the following sixth period transition elements have higher ionization ener- gies than their lighter congeners and the trend persists as far as lead. This results in the metals osmium-mer- cury being the most "noble" of the transition metals. It also gives rise to the "inert pair effect" seen in thal- lium(1) and lead(I1) (see further discussion below). The lanthanide contraction and concomitant increase in effective nuclear charge have more subtle effects which have been examined only recently. For ex- ' -:I 0.50 ample, it appears that the particular balance of contrac- ... -. . - - . . . . tion and overlap of d orbitals compared with the atomic Figure 1. Effective ionic radii of elements of Period Five and Period Six (Coordination Number = 61. Doto from Shannon and Prewilt 121. radius2 results in improved pi-bonding ability by the

Figure 1 illustrates the nature of the lanthanide con- Presented at the Southeast-Southwest Combined Regional traction by a comparison of the effective ionic radii of Meeting of the American chemical society, iyew the elements of Periods Five and Six with coordination Louisiana, December, 1970. number six (8). The contraction of the tripositive ions' 'For a comparative plot of atomic radii, see MOELLER,T., across the series of fourteen lanthanide elements is suffi- J. CnEM.EDUC., 479 417 (lQ70). The "size" of an atom, whether expressed as a covalent, ionic, cient to neutralize completely the normal increment in ., d,, Waals radius is a. of the distance st which the size resulting from a change in principal quantum num- re~ulsion~. the &red electrons of atom (ion) A hs those of B her of the valence shell from five to six. As a result, we balances the attractive forces present. 1; the presknt instance, pairs of elements of essentially identical radii the effect of increased effective nuclear charge on the inner, core and properties: eirconium-hafnium, niohium-tantalum, electrons is not necessarily the same as that upon the d orbitals involved in pi-bonding. The difference in the two effectsappears molybdenum-tungsten. The similarities in chemical to increase the strength of the pi bonds in compounds of the properties of these congeneric pairs and the resulting heavier metals.

Volume 49, Number 4, April 1972 / 227 Table 1. Ground State lonization Energies, Effect of the Lanthanide Contraction

Group numbee IA IIA IIIB IVB VB VIB VIlB VIIIB IB IIB 111.4 IVA VA VIA VIIA VIIIA 5th Period (Rb-Pd) - 4.18 5.70 6.38 6.84 6.88 7.10 7.28 7.37 7.46 8.34 7.58 8.99 5.79 7.34 8.64 9.01 10.45 12.13 6th Period (Cs-Pt)- - 3.89 5.21 5.58 7.0 7.897.98 7.88 8.7 9.1 9.0 9.22 10.44 6.11 7.42 7.29 8.42 ... 10.75

Table 2. Magnitude of the Scondide and Lanthanide Contractions. Oxidation state Soandide contraction Lsnthanide contraction Mat 0.25Alv- .+ = l.00A: 0.38

D All ionic radii for coordination number sir (8). b Estimated from extrapolation of oontractian from Ba'+ (r - 1.36 1) toEu'*(~= l.17h.

Table 3. Ground State Ionization Energies, Effect of the Scandide Contraction

- ~-- IA IIA 111.4 IVA VA VIA VIIA VIIIA 3rd Period (Na-Ar) - Figwe 2. Effedive ionic radii of element. of Period Three and Period 5.14 7.65 5.99 8.15 10.49 10.36 12.97 15.76 Four Itoordin~tion Number = 61. Data from Shannon and Prewitt 121. 4th Period (K-Kr) 4.34 6.11 6.00 7.90 9.81 9.75 11.81 14.00 metals in Period Six. For example, the better pi bond- ing in W(C0)s compared with Mo(C0)a has been as- cribed to the effectsof the lanthanide contraction (6). Table 4. Enthalpies of Atomization, Group IVA Halides,MX. Scandide Contraction A similar, though less impressive, contraction is seen upon the filling of a set of d orbitals. The effect of the "scandide contraction" on the +2 and +3 ions (Z) of the first transition series is illustrated in Figure 2. Ligand field effects cause the low-spin ions to be non- elements of Periods Four and Six, respectively, are in spherical and the effectiveradii to be reduced (the effect many ways similar. The elements tend to be smaller is greatest for the low-spin d6 species). This ligand and to have higher ionization energies than would other- field effect is superimposed upon the (presumably) regu- wise have been the case. In both series the thermo- lar effects of the steady increase in effective nuclear dynamic stability of compounds in higher oxidation charge across the series. If the ligand field effects are states is reduced. In Period Six this effect has been discounted, the total contraction for the scandide series termed "the inert s pair," and results in stability of is roughly two-thirds as great as that for the lanthanide thallium(I), lead(II), and bismuth(I1I) compared to contraction for comparably charged ions (Table 2). higher oxidation states. A similar phenomenon in The importance of the lanthanide contraction has arsenic, selenium, bromine, and (?) krypton has not tended to be more strongly emphasized than that of the been given a formal name but is usually referred to as a scandide series because the former is sufficientlylarge to "reluctance to assume the maximum possible oxidation compensate completely for the change in quantum state." Examples are the absence of an arsenic penta- number (e.g., TH, = r.J in contrast to the incomplete chloride (both PC&and SbCls are known), the decreased reduction in the latter (re, > rAl). Nevertheless, the stability of selenium(VI) compounds compared to sul- effect of the scandide contraction on the properties of fur(VI) and tellurium(VI), and a supposed lessened sta- the transition metals and, especially, on the post-scan- bility of bromine and krypton in their highest oxidation dide elements is significant. The ground state ionization states. An interesting discussion of these phenomena is energies (6) of the main group elements of Periods Three given by Dasent (7). (Na-Ar) and Four (K-ICr) are listed in Table 3. The The explanation of these phenomena in Periods Four inefficient shielding of the electrons added to the 3d and Six is not completely clear but their parallel nature orbitals results in larger ionization energies for the post- is obvious. Table 4 lists the enthalpies of atomization scandide elements than might have been expected if the (= 4 X average bond energy) of the Group IVA halides. scandide contraction were overlooked. Carbon excluded, the bond energies decrease with in- creasing atomic weight of the metal: Si > Ge > Sn > Reduction of Thermodynamic Stability and Pb. The rate of decrease is not uniform, however, for Increased Eledronegotivity the compounds of germanium and lead form weaker The effects of the scandide contraction and the lan- bonds than might be expected (or alternatively, those of thanide contraction on the properties of the heavier silicon and tin are stronger). There have been two

228 / lournol of Chemical Educafion explanations of these data (8, 9). The reader is re- Table 5. Pro~ertiesof the Grouo lllA Elements ferred to the original work for the complete arguments which may be briefly summarized as follows. Drago (8) assumed Pauling (10) electronegativities (C = 2.5, Si = Ge = Sn = Pb = 1.8). Since the electronegativities of the four heavier elements were assumed to be con- stant Drago concluded that the contribution of ionic res- onance energy was also constant. Hence he believed all of the difference in bonding resulted from poorer overlap Table 6. Prooerties of the Grou~IVA Elements in the heavier elements. This poorer overlap was Ground state Ground state Mulliken-Je.66 thought to result from decreased effectiveness of over- ionination electron valenee state lap in the heavier elements and the increased inner core Element energy affinity e1eotrone.stivity C 11.260 1.17 2.48 (te) repulsions caused by dlo and f" electron configurations. Si 8.151 1.39 2.25 (tel Ge 7.899 1.37 2.50 (te) Allred and Rochow (9) assumed, conversely, that Sn 7.344 1.47 2.44 (tel although the overlap did indeed vary within the group Ph 7 dl6 1 79 ? it was given adequately by the M-M bond energyaand that the variation in bond energies within the group stemmed from differences in ionic resonance energy and, congener that does not have these orbitals. All of the hence, from differences in electronegativity. From heavier congeners will have the same effect from similar their Pauling-type calculations from hond energies as subshells with higher principal quantum numbers and well as various other experimental data, they assigned so the properties should once again vary smoothly (until electronegativity values C > Si < Ge > Sn << Pb. The a new type of orbital is filled). anomalously high values for the electronegativities of Extension to Other Periods germanium and lead can readily be ascribed to the in- creased attraction for electrons by nuclei poorly shielded We might now ask whether this phenomenon is by d10 and f 4 configurations. Drago (11) disputed the limited to the post-scandide and post-lanthanide ele- interpretation of Allred and Rochow but the latter ments. To he sure, the d and f electrons have the low- reaffirmed their point of view (1f?) and discounted that est shielding ability (15), but no electrons shield com- of Drago. pletely and so the filling of any set of orbitals results in The controversy over the proper electronegativities an increased effective nuclear charge. The decrease in of the post-transition elements has not been resolved, size as the p orbitals fill is well-known and has been but Mulliken-Jaff6 electronegativities (IS, 14) based on expressed quantitatively for the tetrahedral covalent ionization energy--electron affinity data (Tables 5, 6) radii of the second period by Beagley (16) indicate that the electronegativities of the Group IIIA and Group IVA elements are far from regular. Un- fortunately, the spectroscopic data necessary to calcu- late the valence state energies of the Period Six elements Although this decrease in atomic radius is well-known, (Tl, Pb) are not available but the Period Four elements the logical extension to the properties of the post-p6 are clearly more electronegative than their congeners elements does not seem to have been made previously. immediately above and below them in the family. In From the behavior of po~t-d'~and post-f14 elements we view of the higher ground state ionization energies and might expect decreased stability in the compounds of electron affinities of thallium and lead (compared to the first elements following a completed set of p orbitals. indium and tin), it would be surprising if their valence These elements are sodium and magnesium and, indeed, state values and hence their electronegativities were not the halides of these elements exhibit lower enthalpies of higher as well. Although the controversy is not com- atomization (17, 18) than their lighter and heavier pletely resolved4it would appear that .4llred and Rochow congeners (Tables 7 and 8). The reduced stability of (9, 18) were at least qualitatively correct in their in- the sodium and magnesium halides has been analyzed terpretation of the electronegativities of the elements of in considerable detail with respect to the effects of varia- Groups IIIA and IVA. The increased effective atomic tion in electronegativity, size, and ionic versus covalent number of elements following the filling of poorly shield- contributions to the bonding (19) but for the present ing subshells raises the effective electronegativities of discussion we may interpret it as a p orbital analog of these elements and reduces the stability of their com- pounds in higher oxidation states and with more elec- Table 7. Enthalpies of Atomization, Alkoli Halides tronegative elements. This decrease may be of the F CL Br I order of only a few kilocalories per mole or so but the Li 137.5 111.9 100.2 84.6 phenomenon appears to he a recurring one. The effect Nh 114.0 97.5 86.7 72.7 K 117.6 101.3 '30.9 76.8 will be most notable after theJirst filling of the new type Rb 116.1 100.7 90.4 76.7 of orbitals because the comparison is then with a lighter CS 119 6 106.2 98.5 82.4

Table 8. Enthalpies of Atomization, Alkaline Earth Halides "he value of the Ph-Pb hond is unknown and Allred and F C1 Br I Roehow were forced to use Pauling's second equation for ionic resonance energy, A = AH,/% See ref. (lo),pp. 91-94. Further uncertainty comes in choosing the proper ualence stale for the tetravalent metals: should the hybridization be taken as spa or should d orbit& be included?

Volume 49, Number 4, April 1972 / 229 the lanthanide and scandide effects. In addition, the Table 9. Halide Ionic Radii ond Noble Gas reduced stability of rubidium and strontium compounds Van der Waals Radii can be viewed as another post-dl0 effect. Similar post- period 7.-(AP ntA~b f'4 effects may also be predicted for the compounds of francium and radium. Having observed the effect of completion of subshells for p, d, and j orbitals, we now turn to the only other type of orbital filled in the known elements, the s orbi- tal. Since the first s orbital is fillcd almost immediately at the second element, helium, it is difficult to discern fects. The "super-actinide" contraction should be the trends that may differin the post-s2 elements. May we largest known and it would not be surprising if dvi- suggest, only partially facetiously, that the well-known thallium and dvi-lead were smaller than eka-thallium uniqueness of hydrogen and the less well-known unique- and eka-1ead.O Almost certainly they will ba more ness of helium may be related to this phenomenon-all electronegative than their lighter congeners. elements except H and He are post-s2 elements! Since the 1s shell fills at helium and the first p subshell (2p) Summary fills only eight elements later at neon, it is difficult to separate the effects of the two types of orbital-filling. It In summary, we may say that the effects of the lan- would appear that the filling of the 1s orbital at helium thanide contraction, long known to be of considerable has a rather small effect compared with the filling of the heuristic and pedagogical value, are of even greater 2p orbital at neon. The relatively small increase in importance than has previously been supposed. Ra- effective atomic number in this first series does not pro- dius contraction, increased electronegativity, and re- duce a contraction as large as those seen in later series. duced stability of compounds with more electronega- This produces an intcresting size effect difference be- tive elements seem to accompany post-s2, -pa, -dlo, and tween the hydride ion and the halide ions and between -f14 elements and may be expected for post-g18 elements hdium and thc remainder of the noble gases (Table 9). as well. Such effects adequately account for various In addition, the latter radii illustrate the dl0effect, some- alternations in properties within a given family of the what diluted, in the similarity in size of the elements to periodic chart, and may be easily overlooked in the Period Four to those of Period Three. Although we do usual cursory examination of periodic effects. . not have data for the astatide ion or radon, we may ex- Literature Cited pect their radii to show the effects of an f" configuration (11 KELLEB.R. N., J. Cnm. Eonc., 28, 71 (1951). and be only slightly larger than the elements of Period (2) SXANNON,R. D.. AND PREWITT.C. T.. Act. CIU~~G~~OYI.,BZ6. 1046 Five. \.".",.r,oin\ (3) MOELLDFC.T., "Inorganic Chemistry," John Wiley & Sons. New York, Finally, we come to dements that are as yet undis- 1952; COTTON,F. A,, AND WIIIKINSON,G.. "Advanced Inorgaoic covered but which may show sufficient stability for Chemistry" (2nd ed). John Wilev & Sons. New York. 1966. (4) JOHNSON,O., J. CXEM.EDUC., 47. 431 (19701. chemical characterization. Predictions have been made (51 MOORE.C. E., "Ioniz~tionPotential8 and Ionizstion Limits Derived from the Aodyses of Optical S~eotra;' National Bureau of Standard8 concerning certain physical and chemical properties of Bulletin, NSRDDS-NRS 34. 15'70. elements 113 and 114 (20) and 118 (21). These ele- (6) Kma. R. B.. Inovg. Nuel. Cham. Lett., 5,905 (19691. (7) DAEENT,W. E.. "Nonexistent Compounds." Mhroel Dekker, New ments are expected to have lower electronegativities York. 1965,pp. 117-124. than their lighter congeners and 113 and 114 are ex- (81 DRAOO.R. 8.. J. Phys. Chem., 62,353 (19581. (91 Amnm. A. L., AND ROCHO'N,E. G., J. Inwg. Nud. Chem., 5, 269 pected to show the "inert s pair effect" to an even greater (1958). extent than in thallium and lead (20). (101 P~u~lra,L., "The Nature of the Chemical Bond" (3rd ed.1 Cornell University Press, Ithaoa, New York, 15'60. In contrast, the heavier congeners, dvi-thallium (163) (111 Dnmo, R. S., J. Inow. Nuel. CAem., 15,237 (19601. (12) ALLXSD,A. L., AND Rocxow, E. G., J. Inoro. Nucl. Cham., 20, 167 and dvi-lead (164), should they have half-lives suffi- \."".,.ilClCII ciently long for study (22), may be expected to differ (13) MUL~~KEN,R. S., J. Chem. Phsa.. 2, 782 (1934): 3, 573 (19351; Mo~rrrr,W., Pmc. Roy. Sor. Ser. A. 202, 548 (19501. considerably. They will be among the first post-g18 (14) H~azm,J., AND JhrrB, H. H., J. Amcr. Chem. Soc., 84, 540 (19621; elements, the post-superactinide element^.^ Since the J. Phys. CAem.. 67, 1501 (1963); HINZE.I., AND JAPF*. H. H., J. Amai. Cham. Soc., 85. 148 (1963). effects of the first completion of a subshell appear to (15) SLATEX,J. C.. "Theory of Atomic Structure," Vol. I. MeGraw-Hill. increase s2 p6 dl0 fl$ we might expect the first New York, 1960. < < < (16) BEADLEI,E., Chem. Commun., 388 (19661. post-g18 elements to show some rather remarkable ef- (17) BREWLR,L., AND BRACHETT,E.. Chem. Rw., 61, 425 (19611. 118). . BREWER.L.. SOMAYAJO~U.G. R., AND BRACRETT,E., Chcm. Re"., 61, 111 (1963). (19) EVANB,R. S., AND HUXEEY,J. E.,J. Inoro. Nuel. Chem., 32,777 (1970). (20) KELLEA.JR.. RORNETT.J. L., CARLSON,T. A,. AND NEBTOR,JR., 6 For one version of the extended periodic chart illustrating 0. L.. C. W., J. Phys. Chem., 74, 1127 (1970). these elements see Figure 1 of Seaborg (29). (21) Gnoss~,A. V., J. Inow. Nucl. Chem., 27, 509 (1965). It would, of course, be most satisfactory to compare the (22) Smnono, G.T.. J. Chem. Edu., 46, 626 (1968). radius of a. g1 species with that of s. g18 species, but current pre- (23) WABEB.J. T., Cno~~n,D. T., AND Lrssn~~~,D., J. Chcm. Phgs., 51, (93) as 664 (1969); Gnmwen. W.. AND Fmcae. B.. Phva. Lclt.. 30B, 317 dictions indicate that a consequence of spin-orbit coupling (1969): MANN,J. B., nm W~sen,J. T., J. Chem. Phya., 53, 2397 and the closely spaced energy levels there will be no simple g'-g'8 (1970): Fmcxm, B., Gnrrnm, W., AND W~sen,J. T., Theoret. series analogous to the scandide and lanthanide series. Chim. Aela (Berl.) 21, 235 (19711.

230 / Journol of Chemical Education