The elements which constitute Group 1 of the Periodic Table are known as the alkali metals. They are lithium Li, sodium Na, potassium K, rubidium Rb, cesium Cs, and francium Fr. (Sometimes the NH4+ ion is included among these since it resembles K+ or Rb+ in many of its reactions.) All six of the elements have atoms characterized by an outer electron structure of ns1 with n representing the principal quantum number. The elements exhibit marked resemblances to each other with Li deviating the most. This deviation is assignable to the small size of Li which causes the positive charge of Li+ to be concentrated, that is, the charge density is high. All of the elements exhibit oxidation numbers of 0 and I, with exceptions being rare, such that their chemistries are dominated by the oxidation state I. The six metals are exceptionally reactive, being strong reductants, reacting with HOH at all pH values to give H2 and M+, and having hydroxides MOH which are strong and soluble. Ionic sizes in pm for the members of the group are as follows: Li (76), Na (102), K (139), Rb (152), Cs (167), and Fr (180). The E° values for the M+/M couples are as follows: Li (−3.04 v), Na (−2.71 v), K (−2.93 v), Rb (−2.92 v), Cs (−2.92 v), and Fr (about −3.03 v). a. E–pH diagram. The E–pH diagram for 10−1.0 M Li is presented in Figure 5.1. The figure legend provides an equation for the line that separates Li+ and Li. The horizontal line appears at an E value of −3.10 v. Considerably above the Li+/Li line, the HOH ≡ H+/H2 line appears, which indicates that Li metal is unstable in HOH, reacting with it to produce H2 and Li+. Note further that Li+ dominates the diagram reflecting that the aqueous chemistry of Li is largely that of the ion Li+.
Aqueous Chemistry of Element 105
