The Beryllium Group
The Beryllium Group
The elements which constitute the Be Group of the Periodic Table are known as the alkaline earths. They are beryllium Be, magnesium Mg, calcium Ca, strontium Sr, barium Ba, and radium Ra. All six of the elements have atoms characterized by an outer electron structure of ns2 with n representing the principal quantum number. The elements exhibit marked resemblances to each other with Be differing considerably. This deviation is assignable to the small size of Be which causes the positive charge of Be+2 to be concentrated, that is, the charge density is high. The higher charge-density ions attack HOH to attach OH− and to liberate H+, that is, they hydrolyze readily. All of the elements exhibit oxidation numbers of 0 and II, with their chemistries being dominated by the oxidation state II. The six metals are exceptionally reactive, being strong reductants, reacting with acids, HOH, and bases at all pH values to give H2. The other product of such reactions are M+2 ions and M(OH)2, the ions being present at lower pH values and the hydroxides being present at higher pH values. The transition from M+2 to M(OH)2 occurs at increasing pH values from Be to Ra, such that the hydroxide of Sr is slightly soluble and those of Ba and Ra are soluble. These soluble hydroxides are strong bases. Ionic sizes in pm for the members of the group are as follows: Be (59), Mg (72), Ca (100), Sr (132), Ba (135), and Ra (148). The E° values for the M+2/M couples are as follows: Be (−1.97 v), Mg (−2.36 v), Ca (−2.87 v), Sr (−2.89 v), Ba (−2.91 v), and Ra (−2.91 v), indicating that they are very reactive metals. a. E–pH diagram. The E–pH diagram for 10−1.0 M Be is presented in Figure 6.1. In the figure legend are equations which describe the lines separating the species. It can be seen that Be is thermodynamically unstable with respect to H+, HOH, and OH−. However, Be is relatively inactive in the middle pH range due to a protective oxide coat. To dissolve Be, dilute acids such as HCl or H2SO4, or bases such as NaOH or KOH are required.
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