Mendelevium behavior

The transeinsteinium actinides, fermium (Fm), mendelevium (Md), nobelium (No), and lawrencium (Lr), are not available in weighable (> ng) quantities, so these elements are unknown in the condensed bulk phase and only a few studies of their physicochemical behavior have been reported. Neutral atoms of Fm have been studied by atomic beam magnetic resonance 47). Thermochromatography on titanium and molybdenum columns has been employed to characterize some metallic state properties of Fm and Md 61). This article will not deal with the preparation of these transeinsteinium metals. 

When the divalent state of Md was first discovered, extraction chromatography was used to prove that the behavior of Md + was dissimilar to that of Es + and Fm + (20). The extractant, bis(2-ethylhexyl)phosphoric acid (HDEHP), has a much lower affinity for divalent ions than it does for the tri- and tetravalent ones. Thus, the extraction of Md2+ is much poorer than the extraction of the neighboring tripositive actinides as indicated by the results shown in Table 2. This became the basis for a separation method in which tracer Md in 0.1 M HC1 is reduced by fresh Jones Reductor in the upper half of an extraction column containing HDEHP absorbed on a fluorocarbon powder in the lower half. Mendelevium, in the dipositive state, is rapidly eluted with 0.1 M HC1 whereas the other actinides are retained by the extractant. The separation is quickly performed, but the Md contains small amounts of Zn2+ from the Jones Reductor and also Eu2+, which was added prior to the elution to prevent reoxidation of Md2+ by the extractant. 

Table 2. Comparison of the extraction behavior of tracer einsteinium, fermium, and mendelevium after treatment with various reducing agents. The column-elution method of extraction chromatography was used with the extractant HDEHP adsorbed on a column bed of a fluoroplastic powder (20)...

Before its discovery, the trivalent state was predicted to be the most stable in aqueous solution and, therefore, Md was expected to exhibit a chemical behavior similar to the other 3 + actinides and lanthanides [43]. The elution of Md just prior to Fm in the elution sequence of trivalent actinides from a cation-exchange column observed in the discovery experiments appeared to confirm this prediction. Mendelevium forms insoluble hydroxides and fluorides that are... 

In 1973, Mikheev et al. reported that a stable, monovalent Md ion could be produced in ethanol solutions and that it co-crystallized with CsQ [45]. However, Samhoun and co-workers studied the overall reduction of Md to the amalgam using controlled potential electrolysis they concluded that Md could not be considered a cesium-like element and no evidence was obtained consistent with a monovalent state [46,47]. Hulet et al. have recently repeated some of the co-crystallization experiments of Mikheev and performed a series of new experiments in an attempt to prepare Md by reduction with Sm in ethanol solutions and also in fused KQ media [37]. In these experiments, the behavior of Md was compared to tracer amounts of 3-i-, 2-i-, and 1-i-ions and Md consistently followed the 2 -i- ions. They concluded that Md cannot be reduced to a monovalent state with Sm as daimed by Mikheev. However, on the basis of the results of thermodynamic studies of the co-crystallization process of mendelevium with chlorides of alkali metals, the Russian investigators maintain that Md can be reduced to the monovalent state in water-ethanol solutions and that the co-crystallization of Md with salts of divalent ions can be explained as being due to the formation of mixed crystals [102,103]. An ionic radius of 1.17 A was calculated for Md from the results of the co-crystallization studies [104]. 

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