The standard electrode potentials of Ca and Sr are not very different from that of Ba, indicating that the opposing trends in ionization energies and hydration energies are of roughly equal importance. In contrast to the alkali metals, the heaviest alkaline earth metal (Ba) is the strongest reductant, and the lightest (Be) is the weakest. For example, the density of Ca is less than that of Be and Mg, the two lightest members of the group, and Mg has the lowest melting and boiling points. The group 2 elements do exhibit some anomalies, however. As we would expect, the first ionization energy of an alkaline earth metal, with an ns 2 valence electron configuration, is always significantly greater than that of the alkali metal immediately preceding it. *The values cited are for six-coordinate ions except for Be 2+, for which the value for the four-coordinate ion is given.Īs with the alkali metals, the atomic and ionic radii of the alkaline earth metals increase smoothly from Be to Ba, and the ionization energies decrease. Table 21.4 Selected Properties of the Group 2 Elements The method used by both men was reduction of the chloride by the potent “new” reductant, potassium: In 1808, Davy was able to obtain pure samples of Mg, Ca, Sr, and Ba by electrolysis of their chlorides or oxides.īeryllium (Be), the lightest alkaline earth metal, was first obtained in 1828 by Friedrich Wöhler in Germany and simultaneously by Antoine Bussy in France. The ashes that did not melt (but did dissolve in acid), originally called alkaline earths, were subsequently identified as the alkaline earth oxides (MO). Medieval alchemists found that a portion of the ashes would melt on heating, and these substances were later identified as the carbonates of sodium and potassium (M 2CO 3). In fact, the name alkali comes from the Arabic al-qili, meaning “ashes,” which were known to neutralize acids. In contrast to the alkali metals, however, compounds of the alkaline earth metals had been recognized as unique for many centuries. Four of the six group 2 elements-magnesium (Mg), calcium (Ca), strontium (Sr), and barium (Ba)-were first isolated in the early 19th century by Sir Humphry Davy, using a technique similar to the one he used to obtain the first alkali metals. Because they form +2 ions that have very negative reduction potentials, large amounts of energy are needed to isolate them from their ores. Like the alkali metals, the alkaline earth metals are so reactive that they are never found in elemental form in nature.
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