The Alkaline Earth Metals (Group 2)Skills to Develop. To describe how to isolate the alkaline earth metals. To be familiar with the reactions, compounds, and complexes of the alkaline earth metals. Like the alkali metals, the alkaline earth metals are so reactive that they are never found in elemental form in nature. Basic Biostatistics is a concise, introductory text that covers biostatistical principles and focuses on the common types of data encountered in public health and biomedical fields. ClassZone Book Finder. Follow these simple steps to find online resources for your book. Because they form +2 ions that have very negative reduction potentials, large amounts of energy are needed to isolate them from their ores. Four of the six group 2 elements—magnesium (Mg), calcium (Ca), strontium (Sr), and barium (Ba)—were first isolated in the early 1. Sir Humphry Davy, using a technique similar to the one he used to obtain the first alkali metals. In contrast to the alkali metals, however, compounds of the alkaline earth metals had been recognized as unique for many centuries. In fact, the name alkali comes from the Arabic al- qili, meaning “ashes,” which were known to neutralize acids. 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. The ashes that did not melt (but did dissolve in acid), originally called alkaline earths, were subsequently identified as the alkaline earth oxides (MO). In 1. 80. 8, Davy was able to obtain pure samples of Mg, Ca, Sr, and Ba by electrolysis of their chlorides or oxides. Beryllium (Be), the lightest alkaline earth metal, was first obtained in 1. Friedrich W. The method used by both men was reduction of the chloride by the potent “new” reductant, potassium: \. Marie Curie was awarded the Nobel Prize in Chemistry in 1. Because of its low abundance and high radioactivity however, radium has few uses. Preparation of the Alkaline Earth Metals. The alkaline earth metals are produced for industrial use by electrolytic reduction of their molten chlorides, as indicated in this equation for calcium: \. Beryl is a gemstone and an important source of beryllium. Chemical reductants can also be used to obtain the group 2 elements. For example, magnesium is produced on a large scale by heating a form of limestone called dolomite (Ca. AP Chemistry Summer Assignment 2016 Prerequisites: AP Chemistry is intended to be a second year chemistry course. It is assumed that prior to taking AP Chemistry, you have successfully completed a Chemistry I course (with at. About Chiral Publishing and An Introduction to Chemistry by Mark Bishop. Teacher Login / Registration : Teachers: If your school or district has purchased print student editions, register now to access the full online version of the book. Once you have registered, you can add books to your profile. CO3. Strontium was obtained from strontianite (\(Sr. CO. The alkaline earth metals are somewhat easier to isolate from their ores, as compared to the alkali metals, because their carbonate and some sulfate and hydroxide salts are insoluble. A crystal of strontianite. Both strontianite, one of the most important strontium ores, and strontium are named after the town of Strontian, Scotland, the location of one of the first mines for strontium ores. General Properties of the Alkaline Earth Metals. Several important properties of the alkaline earth metals are summarized in Table \(\Page. Index. Although many of these properties are similar to those of the alkali metals (Table \(\Page. Index. As we would expect, the first ionization energy of an alkaline earth metal, with an ns.
The group 2 elements do exhibit some anomalies, however. 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. In contrast to the alkali metals, the heaviest alkaline earth metal (Ba) is the strongest reductant, and the lightest (Be) is the weakest. 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. One major difference between the group 1 and group 2 elements is their electron affinities. With their half- filled ns orbitals, the alkali metals have a significant affinity for an additional electron. In contrast, the alkaline earth metals generally have little or no tendency to accept an additional electron because their ns valence orbitals are already full; an added electron would have to occupy one of the vacant np orbitals, which are much higher in energy. Reactions and Compounds of the Alkaline Earth Metals. With their low first and second ionization energies, the group 2 elements almost exclusively form ionic compounds that contain M2+ ions. As expected, however, the lightest element (Be), with its higher ionization energy and small size, forms compounds that are largely covalent. Some compounds of Mg. Hence organometallic compounds like those discussed for Li in group 1 are also important for Be and Mg in group 2. Note. The group 2 elements almost exclusively form ionic compounds containing M2+ ions. All alkaline earth metals react vigorously with the halogens (group 1. MX2). Except for the beryllium halides, these compounds are all primarily ionic in nature, containing the M2+ cation and two X. The beryllium halides, with properties more typical of covalent compounds, have a polymeric halide- bridged structure in the solid state, as shown for Be. Cl. 2. These compounds are volatile, producing vapors that contain the linear X–Be–X molecules predicted by the valence- shell electron- pair repulsion (VSEPR) model. As expected for compounds with only four valence electrons around the central atom, the beryllium halides are potent Lewis acids. They react readily with Lewis bases, such as ethers, to form tetrahedral adducts in which the central beryllium is surrounded by an octet of electrons: \. The solid has a polymeric, halide- bridged structure. Note. Because of their higher ionization energy and small size, both Be and Mg form organometallic compounds. The reactions of the alkaline earth metals with oxygen are less complex than those of the alkali metals. All group 2 elements except barium react directly with oxygen to form the simple oxide MO. Barium forms barium peroxide (Ba. O2) because the larger O2. In practice, only Be. O is prepared by direct reaction with oxygen, and this reaction requires finely divided Be and high temperatures because Be is relatively inert. The other alkaline earth oxides are usually prepared by the thermal decomposition of carbonate salts: \. When the reactants are present in a 1: 1 ratio, the binary chalcogenides (MY) are formed; at lower M: Y ratios, salts containing polychalcogenide ions (Yn. Except for Be. O, which has significant covalent character and is therefore amphoteric, all the alkaline earth oxides are basic. Thus they react with water to form the hydroxides—M(OH)2: \. Hydroxides of the lighter alkaline earth metals are insoluble in water, but their solubility increases as the atomic number of the metal increases. Because Be. O and Mg. O are much more inert than the other group 2 oxides, they are used as refractory materials in applications involving high temperatures and mechanical stress. For example, Mg. O (melting point = 2. Only the lightest element (Be) does not react readily with N2 to form the nitride (M3. N2), although finely divided Be will react at high temperatures. The higher lattice energy due to the highly charged M2+ and N3. Similarly, all the alkaline earth metals react with the heavier group 1. M3. Z2. Note. Higher lattice energies cause the alkaline earth metals to be more reactive than the alkali metals toward group 1. When heated, all alkaline earth metals, except for beryllium, react directly with carbon to form ionic carbides with the general formula MC2. The most important alkaline earth carbide is calcium carbide (Ca. C2), which reacts readily with water to produce acetylene. For many years, this reaction was the primary source of acetylene for welding and lamps on miners’ helmets. In contrast, beryllium reacts with elemental carbon to form Be. C, which formally contains the C4. Consistent with this formulation, reaction of Be. C with water or aqueous acid produces methane: \. All the heavier alkaline earth metals (Mg through Ba) react directly with hydrogen to produce the binary hydrides (MH2). The hydrides of the heavier alkaline earth metals are ionic, but both Be. H2 and Mg. H2 have polymeric structures that reflect significant covalent character. All alkaline earth hydrides are good reducing agents that react rapidly with water or aqueous acid to produce hydrogen gas: \. In this case, however, two solvated electrons are formed per metal atom, and no equilibriums involving metal dimers or metal anions are known. Also, like the alkali metals, the alkaline earth metals form a wide variety of simple ionic salts with oxoanions, such as carbonate, sulfate, and nitrate. The nitrate salts tend to be soluble, but the carbonates and sulfates of the heavier alkaline earth metals are quite insoluble because of the higher lattice energy due to the doubly charged cation and anion. The solubility of the carbonates and the sulfates decreases rapidly down the group because hydration energies decrease with increasing cation size. Note. The solubility of alkaline earth carbonate and sulfates decrease down the group because the hydration energies decrease. Complexes of the Alkaline Earth Metals. Because of their higher positive charge (+2) and smaller ionic radii, the alkaline earth metals have a much greater tendency to form complexes with Lewis bases than do the alkali metals. This tendency is most important for the lightest cation (Be. Note. The alkaline earth metals have a substantially greater tendency to form complexes with Lewis bases than do the alkali metals. The chemistry of Be. Lewis acid, forming complexes with Lewis bases that produce an octet of electrons around beryllium. For example, Be. 2+ salts dissolve in water to form acidic solutions that contain the tetrahedral . Because of its high charge- to- radius ratio, the Be. Hence beryllium oxide is amphoteric. Beryllium also forms a very stable tetrahedral fluoride complex: . Recall that beryllium halides behave like Lewis acids by forming adducts with Lewis bases (Equation \(\ref. Complex formation is most important for the smaller cations (Mg. Ca. 2+). Thus aqueous solutions of Mg. Like the alkali metals, the alkaline earth metals form complexes with neutral cyclic ligands like the crown ethers and cryptands discussed in Section 2.
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