First-member anomaly

But whereas Bent and Jensen have agreed to share the credit for the discovery of this more detailed aspect of first-member anomaly they draw surprisingly different conclusions regarding the noble gases. For Jensen, helium remains a noble... 

In contemporary terms, one might also see the differences between fluorine and the other halogens as resulting from the phenomenon of first-member anomaly, whereby the uppermost element in the main-group elements shows anomalous behavior when compared with other group members. 

Nor can the anomalous values for aluminum be attributed to the phenomenon of first-member anomaly for the simple reason that aluminum is the second member of group 13 as matters currently stand. 

The Anomaly Quite often the first member of a periodic group differs from the remaining members of... 

The existence of an anomaly in the spectrum of Ba+ had been known for many years. It was commented on by Saunders et al. [213], who remarked the perturbation. .. appears to be of a novel type but could advance no explanation for it. The realisation that this is due to a near-critical potential barrier effect came much later [212]. Hartree-Fock calculations were used to establish that the n/ wavefunctions of Ba+ become resonantly localised in the inner well of a double-well potential around n = 5. As a consequence, all the low n orbitals become distinctly bimodal or hybrid in character (as can be seen in fig. 5.12) with peaks in both the inner and the outer potential wells. This results in (i) an unusual course in the quantum defects for the nf series they are not constant, but depend strongly on the energy, despite the absence of any local perturbation (ii) a pronounced intensity anomaly in the 5d — nf excitation series, which was first noted by Roig and Tondello [214] and (iii) an anomaly in the course of the spin-orbit splittings for the first members of the series, both of which can be explained on the basis of the bimodal character of the... 

Figure 8.14 Photoionization cross-section of N2 taken at 77 K with a resolution of 0.016 A (from Dehmer, et al., 1984). The first members of the v = 1 series are vibra-tionally autoionized by the v+ = 0 continuum of Nj X2Sg. For n > 14, anomalies result from indirect electrostatic autoionization (from Giusti-Suzor and Lefebvre-Brion, 1984).

Surprising as it may seem, some chemists have even proposed chemical evidence for placing helium in this manner. Such arguments are based on the first-element rule which, in its simplest form, states that the first element in any group of the periodic system tends to show several anomalies when compared with successive members of its group. For example, in the p block, all the first-member elements show a reluctance to expand their octets of outer-shell electrons, while subsequent group members do so quite readily. ... 

In addition, there is a more sophisticated version of this first-member rule that also specifies the extent to which the first elements in the various blocks of the Periodic Table display anomalies. For example, Jensen writes. 

Refractory materials in primitive meteorites were investigated first as they have the best chance of escaping homogenization in the early solar system. Inclusions in C3 carbonaceous chondrites exhibit widespread anomalies for oxygen and the iron group elements. Only a few members, dubbed FUN (for Fractionated and Unknown Nuclear effects), also display anomalous compositions for the heavy elements. Anomalies in inclusions have generally been connected with explosive or supernova nucleosynthesis. 

The compressibility of the rare earth metals was first investigated up to 40 kbar by Bridgman (see Lawson s review, 1956) for most of the members of the series, and in some cases to about 90 kbar. Since Bridgman s earlier measurements, Stephens (1964) has published compression data on Pr, Eu, Tb, Yb and Sc to 45 kbar and Perez-Albuerne et al. (1966) have reported on the compressibility of Ho, Er and Tm to about 200 kbar (see Drickamer et al., 1966). More recently Liu et al. (1973) and Liu (1975) have measured the compressibility of Tm and Lu respectively in a diamond anvil high pressure X-ray apparatus to several hundred kbar. Syassen and Holzapfel (1975) have reported on the compression of La to 120 kbar. Anomalies corresponding to the phase transitions discussed earlier were noted in some cases in these static compression measurements at the y to a-Ce transition near 7 kbar (ch. 4, section 2.1) and in La at the dhep-fee transition near 25 kbar (Bridgman, see Lawson s review, 1956), in Yb at the fcc-bcc transition near 40 kbar (Stephens, 1964) and in Lu from the hep to Sm-type transition near 230 kbar (Liu, 1975). Bulk moduli evaluated from these data are plotted in fig. 9.11. For Gd, Dy, and Er single crystal elastic constant data and their pressure variation have been obtained (Fisher et al., 1973). 

In SED VII and VIII, reference was made to the possible deleterious effects of low concentrations of anaesthetic agents in the atmosphere of operating rooms. More specifically, the influence on abortion rates and congenital anomalies of children bom to female anaesthetists was described. Desbaumes et al. (2C) measured brominated metabolites of halothane in the urine of 18 members of the operating theatre staff and found a striking difference between women and men (14.59 mg/1 and 3.04 mg/1 respectively). This difference may be explained by the fact that women staff members are more likely to be continuously present in the operating theatre or that they retain halothane more easily in their body fat. Only the first explanation is consistent with the sex difference in fluroxene toxicity found in mice (3), as this toxicity was more marked in males. 

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