James E. Huheey and Caroline L. Huheey University of Maryland College Park, 20742
II
Anomalous Properties of Elements that MOW "Long Periods" of Elements
The phenomenon known as the lanthanide contraction has long been appreciated as an important factor in determining the properties of the lanthanide and post-lanthanide elements. Appreciation of this phenomenon is of obvious heuristic value with regard to research efforts directed towards these elements. In addition there is an implicit pedagogical value that has been discussed at some length (1) and which is included in most descriptive discussionsof these elements. It is the purpose of this paper to examine the physical nature of this phenomenon and extend the concept to other parts of the periodic table.
Effective ionic radii of elements of Period Five and Period Six (Coordination Number = 61. Doto from Shannon and Prewilt 121.
difficulties in their separation are well-documented (3). Less well-explored are the cases in which the lanthanide contraction results in different properties for the postlanthanide elements (4). In general, these may be related to greater effective nuclear charge and attraction for electrons in the heavier elements. Table I lists the ground state ionization energies (6) for the elements rubidium-xenon (Period Five) and cesium-radon (Period Six). The pre-lanthanide elements cesium, barium, and lanthanum have ionization energies less than their lighter congeners as expected for 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 energies than their lighter congeners and the trend persists as far as lead. This results in the metals osmium-mercury being the most "noble" of the transition metals. It also gives rise to the "inert pair effect" seen in thallium(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 example, it appears that the particular balance of contraction and overlap of d orbitals compared with the atomic radius2 results in improved pi-bonding ability by the
Figure 1 illustrates the nature of the lanthanide contraction by a comparison of the effective ionic radii of the elements of Periods Five and Six with coordination number six (8). The contraction of the tripositive ions' across the series of fourteen lanthanide elements is sufficient to neutralize completely the normal increment in size resulting from a change in principal quantum numher of the valence shell from five to six. As a result, we pairs of elements of essentially identical radii and properties: eirconium-hafnium, niohium-tantalum, molybdenum-tungsten. The similarities in chemical properties of these congeneric pairs and the resulting
Presented at the Southeast-Southwest Combined Regional Meeting of the American chemical society, iyew Louisiana, December, 1970. 'For a comparative plot of atomic radii, see MOELLER, T., J. CnEM.EDUC., 479 417 (lQ70). The "size" of an atom, whether expressed as a covalent, ionic, of the distance st which the d,, Waals radius is a. re~ulsion the &red electrons of atom (ion) A h s those of B balances the attractive forces present. 1; the presknt instance, the effect of increased effective nuclear charge on the inner, core electrons is not necessarily the same as that upon the d orbitals involved in pi-bonding. The differencein the two effectsappears to increase the strength of the pi bonds in compounds of the heavier metals.
'
-:I 0.50
... -. . - -
.
.
. .
Figure 1.
.,
~.
Volume 49, Number 4, April 1972
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227
Table 1. VB
Ground State lonization Energies, Effect of the Lanthanide Contraction Group n VIIIB
IA
IIA
IIIB
IVB
VIB
VIlB
4.18
5.70
6.38
6.84 6.88 7 . 1 0
7.28
7.37
3.89
5.21
5.58
7.0
7.88
8.7
7.897.98
u IIB
111.4
IVA
b VA
VIA
e VIIA
e VIIIA
8.99
5.79
7.34
8.64
9.01
10.45
12.13
10.44
6.11
8.42
...
IB
5th Period (Rb-Pd) 7.46 8.34 7.58 6th Period (Cs-Pt)9.1 9.0 9.22
m
Table 2. Oxidation state
b
7.29
0.25Alv-
.+
=
l.00A:
Lsnthanide contraction 0.38
All ionic radii for coordination number sir ( 8 ) . Estimated from extrapolation of oontractian from Ba'+ (r
1 ) t o E u ' * ( ~= l . 1 7 h .
Table 3.
metals in Period Six. For example, the better pi bonding in W(C0)s compared with Mo(C0)a has been ascribed to the effectsof the lanthanide contraction (6).
IA
IIA
111.4
5.14
7.65
5.99
4.34
6.11
Table 4.
-
1.36
Ground State Ionization Energies, Effect of the Scandide Contraction -
Figwe 2. Effedive ionic radii of element. of Period Three and Period Four Itoordin~tion Number = 61. Data from Shannon and Prewitt 121.
-
10.75
Magnitude of the Scondide and Lanthanide Contractions.
Soandide contraction
Mat
D
7.42
-
IVA
~ - VA -
VIA
3rd Period (Na-Ar) 8.15 10.49 10.36 4th Period (K-Kr) 6.00 7.90 9.81 9.75
VIIA
VIIIA
12.97
15.76
11.81
14.00
-
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 nonspherical and the effectiveradii to be reduced (the effect is greatest for the low-spin d6 species). This ligand field effect is superimposed upon the (presumably) regular effects of the steady increase in effective nuclear charge across the series. If the ligand field effects are discounted, the total contraction for the scandide series is roughly two-thirds as great as that for the lanthanide contraction for comparably charged ions (Table 2). The importance of the lanthanide contraction has tended to be more strongly emphasized than that of the scandide series because the former is sufficientlylarge to compensate completely for the change in quantum number (e.g., TH, = r.J in contrast to the incomplete reduction in the latter (re, > rAl). Nevertheless, the effect of the scandide contraction on the properties of the transition metals and, especially, on the post-scandide elements is significant. The ground state ionization energies (6) of the main group elements of Periods Three (Na-Ar) and Four (K-ICr) are listed in Table 3. The inefficient shielding of the electrons added to the 3d orbitals results in larger ionization energies for the postscandide elements than might have been expected if the scandide contraction were overlooked. Reduction of Thermodynamic Stability and Increased Eledronegotivity
The effects of the scandide contraction and the lanthanide contraction on the properties of the heavier 228
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lournol of Chemical Educafion
elements of Periods Four and Six, respectively, are in many ways similar. The elements tend to be smaller and to have higher ionization energies than would otherwise have been the case. In both series the thermodynamic stability of compounds in higher oxidation states is reduced. In Period Six this effect has been termed "the inert s pair," and results in stability of thallium(I), lead(II), and bismuth(I1I) compared to higher oxidation states. A similar phenomenon in arsenic, selenium, bromine, and (?) krypton has not been given a formal name but is usually referred to as a "reluctance to assume the maximum possible oxidation state." Examples are the absence of an arsenic pentachloride (both PC&and SbClsare known), the decreased stability of selenium(VI) compounds compared to sulfur(VI) and tellurium(VI), and a supposed lessened stability of bromine and krypton in their highest oxidation states. An interesting discussion of these phenomena is given by Dasent (7). The explanation of these phenomena in Periods Four and Six is not completely clear but their parallel nature is obvious. Table 4 lists the enthalpies of atomization (= 4 X average bond energy) of the Group IVA halides. Carbon excluded, the bond energies decrease with increasing atomic weight of the metal: Si > Ge > Sn > Pb. The rate of decrease is not uniform, however, for the compounds of germanium and lead form weaker bonds than might be expected (or alternatively, those of silicon and tin are stronger). There have been two
explanations of these data (8, 9). The reader is referred 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 constant Drago concluded that the contribution of ionic resonance energy was also constant. Hence he believed all of the difference in bonding resulted from poorer overlap in the heavier elements. This poorer overlap was thought to result from decreased effectiveness of overlap in the heavier elements and the increased inner core repulsions caused by d l o and f" electron configurations. Allred and Rochow (9) assumed, conversely, that although the overlap did indeed vary within the group 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, hence, from differences in electronegativity. From their Pauling-type calculations from hond energies as well as various other experimental data, they assigned electronegativity values C > Si < Ge > Sn
