1 Chapter The Alkaline Earth Metals

2 Group 2 A on the periodic table

3 Common properties (Except for Be which resembles 3A elements) –Somewhat less electropositive than alkali metals –Less reactive than the alkali metals. –M 2+ ions attain the stable electron configuration of the preceding noble gas –Oxidation number is commonly +2 –All isotopes of radium are radioactive

Beryllium Steel gray Hard High melting point Low density High resistance to corrosion Nonmagnetic 4

Uses gyroscopes windows of X-ray tubes –transparency 5

Sources Bertrandite –Be 4 Si 2 O 7 (OH) 2 Beryl –Be 3 Al 2 Si 6 O 18 –aquamarine –emerald contamination of Cr(III) 6

Chemistry of Beryllium Covalent bonds predominate –high charge density polarizes any anion causing overlap to occur Simple ionic compounds are a mixture –BeCl 2 ·4H 2 O –[Be(OH 2 ) 4 ] 2+ ·2Cl -

Beryllium Metallic, but can form oxyanions –amphoteric –“weak” metal H 2 O(l) + BeO(s) + 2H 3 O + (aq)  [Be(OH 2 ) 4 ] 2+ (aq) H 2 O(l) + BeO(s) + 2OH - (aq)  [Be(OH) 4 ] 2- (aq)

Compound Properties Sweet taste –extremely poisonous Inhalation results in berylliosis

Magnesium Found in many minerals in nature –carnallite KMgCl 3 ·6H 2 O –dolomite MgCO 3 ·CaCO 3 3 rd most common ion in seawater –10 8 million tons

11 Magnesium –Magnesium is the sixth most plentiful element in Earth’s crust (about 2.5 percent by mass). –principal magnesium ores are brucite [Mg(OH) 2 ], dolomite (CaCO 3. MgCO 3 ) and epsomite (MgSO 4. 7H 2 O). –Seawater is a source of magnesium—there are about 1.3 g of magnesium in each kilogram of seawater. –Metallic magnesium is obtained by electrolysis, from its molten chloride, MgCl 2

Isolation of Magnesium Precipitation reaction Ca(OH) 2 (s) + Mg 2+ (aq)  Ca 2+ (aq) + Mg(OH) 2 (s) Neutralization reaction Mg(OH) 2 (s) + 2HCl(aq)  MgCl 2 (aq) + 2H 2 O(l) Placed in a Downs cell Mg 2+ (MgCl 2 ) + 2e -  Mg(l) 2Cl - (MgCl 2 )  Cl 2 (g) + 2e -

Magnesium Uses 4 x 10 5 tons produced annually –½ is used in aluminum-magnesium alloys very low density (1.74 g/ml) used in aircraft, railroads, ships, etc…

Magnesium Chemistry Decomposition upon heating MgCl 2 ·H 2 O(s) + heat  Mg(OH)Cl(s) + HCl(g) Formation of covalent bonds –Grignard reagents C 2 H 5 Br(ether) + Mg(s)  C 2 H 5 MgBr(ether)

15 –Strongly basic hydroxide –Reactions of magnesium –Magnesium is essential to plant and animal life

Magnesium Reaction Flowchart

Calcium and Barium grayish metals react with oxygen 2Ca(s) + O 2 (g)  2CaO(s) 2Ba(s) + O 2 (g)  2BaO(s) Ba(s) + O 2 (g)  BaO 2 (g)

Calcium and Barium strong absorbers of x-rays –bones –dyes CaCl 2 ·2H 2 O(s) + heat  CaCl 2 (s) + 2H 2 O(g) BaCl 2 ·2H 2 O(s) + heat  BaCl 2 (s) + 2H 2 O(g)

Oxides Formed upon reaction with air or heating of the carbonate 2Mg(s) + O 2 (g)  2MgO(s) CaCO 3 (s) + heat  CaO(s) + CO 2 (g)

Oxides CaO –called quicklime –undergoes thermoluminescence

Oxides CaO –reacts with water to form slaked lime CaO(s) + H 2 O(l)  Ca(OH) 2 (s) –used as a neutralizing agent in gardening, along with CaCO 3 Ca(OH) 2 (s) + 2H + (aq)  Ca 2+ (aq) + 2H 2 O(l) CaCO 3 (s) + 2H + (aq)  Ca 2+ (aq) + H 2 O(l)

Calcium Carbonate Two naturally occurring crystalline forms –calcite Iceland spar two refractive indices –aragonite

Calcium Carbonate Chemistry involved in the formation of caves, stalagmites, and stalactites CaCO 3 (s) + CO 2 (aq) + H 2 O(l)  Ca 2+ (aq) + 2HCO 3 - (aq) Ca(HCO 3 ) 2 (aq)  CaCO 3 (s) + CO 2 (g) + H 2 O(l)

Calcium Carbonate Biological importance –dietary supplement helps reduce osteoporosis –antacid –constipative

Portland cement 26% dicalcium silicate –Ca 2 SiO 4 51% tricalcium silicate –Ca 3 SiO 4 11% tricalcium aluminate –Ca 3 Al 2 O 6 2Ca 2 SiO 4 (s) + 4H 2 O(l)  Ca 3 Si 2 O 7 ·3H 2 O(s) + Ca(OH) 2 (s)

Calcium Chloride White, deliquescent solid Formation of CaCl 2 ·6H 2 O is very exothermic (  H = -82 kJ/mol) –used in hot packs

Calcium Chloride Concentrated solutions –used to melt ice lowers m.p. to -55°C –used to coat unpaved roads minimizes dust –used to fill tires of earth-moving equipment better traction

Biological Aspects Photosynthesis –magnesium is contained in chlorophyll and keeps the molecule in a specific configuration 6CO 2 (g) + 6H 2 O(l)  C 6 H 12 O 6 (aq) + 6O 2 (g)

Biological Aspects Magnesium is concentrated inside cells –triggers the relaxation of muscles Calcium is concentrated outside cells –important in blot-clotting –trigger the contractions of muscles

30 Calcium –Earth’s crust contains about 3.4 percent calcium by mass. –Calcium occurs in limestone, calcite, chalk, and marble as CaCO 3 ; in dolomite as CaCO 3. MgCO 3, gypsum as CaSO 4. 2H 2 O; and in fluorite as CaF 2 –Metallic calcium is best prepared by the electrolysis of molten calcium chloride (CaCl 2 ).

31 Reactions of calcium and calcium compounds Metallic calcium serves mainly as an alloying agent Essential for living systems

32 Aluminum Most abundant metal and the third most plentiful element in Earth’s crust (7.5 percent by mass). Elemental form does not occur in nature Principal ore is bauxite (Al 2 O 3. 2H 2 O) Other minerals containing aluminum are orthoclase (KAlSi 3 O 8 ), beryl (Be 3 Al 2 Si 6 O 18 ), cryolite (Na 3 AlF 6 ), and corundum (Al 2 O 3 ) Aluminum used to be considered a precious metal until Hall developed a method of aluminum production.

33 Preparation – Electrolytic production of aluminum based on the Hall process

34 Reactions of the element –Amphoteric –reacts with acid or base –Oxide formation –With metal oxides

35 Aluminum chloride –Exists as a dimer – Al 2 Cl 6

36 Aluminum hydrides –well-defined series of compounds –aluminum hydride is a polymer in which each aluminum atom is surrounded octahedrally by bridging hydrogen atoms

37 Other important reactions –Hydrolysis –Amphoterism –Formation of alums

Chemistry In Action: Recycling of aluminum 38 popularity in the industry: nontoxic, odorless, tasteless, and lightweight Beverage cans were unknown in 1960