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Chapter 12

CHMB3 Chapter 12

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Department
Chemistry
Course
CHMB31H3
Professor
Alen Hadzovic
Semester
Fall

Description
Chapter 12: The Group 2 Elements The Essentials • Alkaline Earth Metals: calcium, strontium, barium and radium, but applied to all of Group 2. • The most important factors influencing the chemical properties of the Group 2 elements are their ionization energies and ionic radii. o Group 2 elements have a greater mechanical hardness and higher melting points because of an increase in the strength of metallic bonding from Group 1 to 2. o The ionization energies of the elements decrease down the group as the radius increases and the elements become more reactive and more electropositive as it becomes easier to form the +2 ions. o M(s) + 2 H2O(l)  M(OH)2(aq) + H2(g) o All elements occur as hexagonal close-packed structures with the exception of Ba and Ra, which adopt the more open body-centered cubic structure. o The density decreases from Be to Mg to Ca due to strong metallic bonding. • The binary compounds of the Group 2 metals contain the cations of the elements and exhibit predominantly ionic bonding. o Except Be, the Group 2 elements form ionic hydrides; the anion present is the hydride ion, H-. o Beryllium hydride adopts a 3-D network of linked Beh4 tetrahedra. o The fluorides of the larger cations (Ca to Ba) adopt the (8,4)-coordinate fluorite structure but MgF2 crystallizes with a rutile structure. o Beryllium oxide, BeO, has a wurtzite structure with (4,4)-coordination and the oxides of the other Group 2 elements all adopt the rock-salt structure with (6,6)-coordination. o BaO(s) + H2O(l)  Ba2+(aq) + OH-(aq); same as SrO. o Be(OH)2(s) + 2 OH-(aq)  Be(OH)4^2-(aq) o The elements Mg-Ra react directly with nitrogen when heated to produce the nitrides M2N3, which react with water to produce ammonia. o In general, the salts of the Group 2 elements are generally much less soluble in water than those of Group 1 on account of the higher lattice enthalpies of structures containing doubly charged cations. • The small size of Be2+ and its consequent high charge density and polarizing power results in the compounds of Be being largely covalent; the ion is a strong Lewis acid. • Beryllium typically has a coordination number of 6 or more. o A significant covalent contribution to the bonding in compounds such as beryllium halides BeCl2, BeBr2 and BeI2 and the hydride, BeH2. o A greater tendency to form complexes, with the formation of molecular compounds such as Be4O(O2CCH3)6. o Hydrolysis of beryllium salts in aqueous solution, forming species such as [Be(OH2)3OH]+ and acidic solutions; hydrated beryllium salts tend to decompose by hydrolysis reactions, where beryllium oxo or hydroxo salts are formed, rather than by the simple loss of water. o The oxide and other chalcogenides of Be adopt structures with the more directional (4,4)-coordination strucutres. o Beryllium forms many stable organometallic compounds like (Be(CH3)2) and ((C5H5)2Be). • Be has a strong diagonal relationship with Al. o Both Be and Al form covalent hydrides and halides; the analogous compounds of the other Group 2 elements are predominantly ionic. o The oxides of Be and Al are amphoteric whereas the oxides of the rest of the Group 2 elements are basic. o In the presence of excess OH- ions, Be and Al form [Be(OH)4]^2- and [Al(OH)4]^2-; no equivalent chemistry is observed for Mg. o Both elements form structures based on linked tetrahedral: Be forms structures built from [BeO4]^n- and [BeX4]^n- tetrahedral (X = halide) and Al forms numerous aluminates and aluminosilicates containing the [AlO4]^n- unit. o Both elements form carbides that contain the C^4- ion and produce methane on reaction with water; the other Group 2 carbides contain the C2^2- ion and produce ethyne on reaction with water. o The alkyl compounds of Be and Al are electron-deficient compounds that contain M-C-M bridges. The Detail • Magnesium is the only Group 2 element extracted on an industrial scale; magnesium, calcium, strontium and barium can be extracted from molten chloride. o CaO(s) + H2O(l)  Ca2+(aq) + 2OH-(aq) o Mg2+(aq) + 2 OH-(aq)  Mg(OH)2(s) o Mg(OH)2(s) + 2HCl(aq)  MgCl2(aq) + 2 H2O(l) • Magnesium and its compounds have major applications in pyrotechnics, alloys and common medicines; calcium compounds are widely used in the construction industry; magnesium and calcium are very important for biological function. o Beryllium is unreactive in air so it is resistant to corrosion; it is also one of the lightest metals so people use in alloys to make precision instruments and missiles. o Magnesium metal burns in air with an intense white flame so it is used in fireworks. o Calcium oxide is a major component of mortar and cement. • All the Group 2 elements form saline hydrides with the exception of beryllium, which forms a polymeric covalent compounds. o MgH2(s) + 2 H2O(l)  Mg(OH)2(s) + 2 H2(g) o Mg(s) + H2(g)  MgH2(s) • The halides of beryllium are covalent; all the fluorides, except BeF2 are inso
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