The Group 2 Elements Be, Mg, Ca, Sr, Ba, Ra

Some pertinent data for the elements are given in Table 4-1. Beryllium has unique chemical behavior with a predominantly covalent chemistry, although it forms an aqua ion [Be(H20)4]2+. Magnesium has a chemistry intermediate between that of Be and the heavier elements, but it does not stand in as close relationship with the predominantly ionic heavier members as might have been expected from the similarity of Na, K, Rb, and Cs. It has considerable tendency to covalent bond formation, consistent with the high charge/radius ratio. For instance, like beryllium, its hydroxide can be precipitated from aqueous solutions, whereas hydroxides of the other elements are all moderately soluble, and it readily forms bonds to carbon. 

All the M2+ ions are smaller and considerably less polarizable than the isoelec-tronic M+ ions. Thus deviations from complete ionicity in their salts due to polarization of the cations are even less important. However, for Mg + and, to an exceptional degree for Be2+, polarization of anions by the cations does produce a degree of covalence for compounds of Mg and makes covalence characteristic for Be. Accordingly, only an estimated ionic radius can be given for Be2+ the charge/radius ratio 

All isotopes of radium are radioactive, the longest-lived isotope being 226Ra (a —1600 years). This isotope is formed in the natural decay series of 238U and was first isolated by Pierre and Marie Curie from pitchblende. Once widely used in radiotherapy, it has largely been supplanted by radioisotopes made in nuclear reactors. 

The elements zinc, cadmium, and mercury, which have two electrons outside filled penultimate d shells, are classed in Group 12. Although the difference between the calcium and zinc subgroups is marked, zinc, and to a lesser extent cadmium, show some resemblance to beryllium or magnesium in their chemistry. We discuss these elements separately (Chapter 15), but note here that zinc, which has the lowest second ionization enthalpy in the Zn, Cd, Hg group, still has a value (1726 kJ mol-1) similar to that of beryllium (1757 kJ mol 1), and its standard potential (-0.76 V) is considerably less negative than that of magnesium. 

There are a few ions with ionic radii and chemical properties similar to those of Sr2+ or Ba2+, notably those of the +2 lanthanides (Section 19-13), especially the europium ion Eu2+, and its more readily oxidized analogues, Sm2+ and Yb2+. Because of this fortuitous chemical similarity, europium is frequently found in Nature in Group 2 minerals, and this is a good example of the geochemical importance of such chemical similarity. 

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