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.