The "Anomalous" Electron Affinity of Lead An Example of lsoelectronic Behavior Jose Antonio Chamizo' Universidad Nacional Autbnoma de Mexlco. 04510 MBxico D.F., Mexico The electron affmity EA of an atom A, sometimes c d e d the Some Energy T e r m (eV) for Group IVA Elements zeroth ionization potential (I), has been defined as the dif- Element SpiMrblt Ionization Elecbon Xz bond ference between the total energies of the ground state of A and X 3Pr3P0 Potential Affinity Do its negative ion A- (2) C 0.005 11.26 1.27 6.1 EA = EA - EA- Si 0.03 8.15 1.39 3.2 Recently the EA of lead was calculated by the technique of laser photoelectron spectrometry (LPES) (3) and was found to he 0.365 * 0.008 eV, a value very different from those re- ported earlier in the literature (1.1 eV (2) and 1.05 * 0.09 eV n-5 Electron Figure 1. Ionization potential and electron affinity of the elements 01 groups 111, IV, and VB. 0.3." 0.37.V 095.V 'SX 'P TI- P b' (4)). With this new value the periodicity of the EA in the sixth row differs from that of the rest of the representative elements in the same way that the periodicity of the ionization potential IP differs (Fie. 1). The anomajbus behavior of the IP of bismuth was discussed bv Smith (5 I in ierms of s~in-orbit cou~linr. The ex~lanation i'hasicall'; the same forthe EA of lead, since both species involved are isoelectronic (Fig. 2). That is, the configuration of the following species is the same: Ph, Bi+ (6p2)and Ph-, Bi (603). l'l;e large stabilization of the ground state "0 (see tahle) oithe 6p2configurationand the negligible stnhilizationof the 'S.< 2 state for the 6p' configuration explain both the anom- alous behavior of the 11' of Bi and the EA of Pb. The inter- pretation of EA as the ionization potential of the anion also facilitates understanding of these phenomena. It is important to consider the relativistic effects on chemical properties, particularly in the case of heavy metals. Some of their anomalous properties, like the yellow color of gold, the liquid state of mercury at STP conditions, the inert pair effect, the greater IP of sixth-row elements compared with their fifth-row analogues, and the large EA of Tl (see Fig. I), can he explained hy these relativistic effects (643). Literature Cited (1) Brmhi, D . W., Meyers, E. A, Sicilia, F.. and Nearing.J. C. J. CHEM EDUC., 50,487 (1973). (2) Hotop, H.. and Linebegor. W . C., J Phys. C h m . Ref Dota, 4,539 (1975). agerl., C. 8, Carderman. R. R, and Lincbsrger, W. C., J. Cham Phya., 74,1513 (3) ;lsaj\ , . - - - , . (4) Chen, E. C. M., and Went-uorth, W . E., J. C m . Eouc.,52,4% 11975). (5) 8mith.D. W., J. CHEM . EDuc., 62,576 (1975). (6) Pit~er. K. S.. Acc. Chem. Re$., 12,271 (19791. (7) Pyykko,P., and Desclsux, J. P., Acc. Chm RPJ, 12.276 (19791. IS) Cruz. D., Chamim, J. A,, Ggnitz. A., "Esvueturs At6mies: Fando Educstivo Inter- amerieanoM€rieo (in mess). Figure 2. Effect of spin-wbit coupling on Me electron affinity of TI. Pb, and Bi ' Present address: School of Molecular Sciences. University of (not drawn to scale). Sussex, Brighton. Sussex, EN1 9Q.J England. 874 Journal of Chemical Education