Ch 8 Main Group Elements continued

1. Ch 8 Main Group Elements continued • Group IVA • The Elements • Sn and Pb are metals known since antiquity • Sn alloy with Cu is Bronze (5000 yrs ago) • Pb used in Egyptian pottery glazes and Roman plumbing systems • Si is a metalloid with long-known oxide: Flint = SiO The free element was identified by Berzelius in 1823 • Ge is metalloid that was identified as “missing” by Mendeleev in 1871 Discovered in 1886 by Winkler

2. C is a nonmetal known as charcoal and diamond, not as an element until 1700’s • Isotopes: • 12C 98.9% is main component of “organic compounds” • 13C 1.1% is an NMR active isotope • 14C 1.2 x 10-10% is formed in the atmosphere and is radioactive • Radiocarbon dating - 14C is incorporated into living things at the same ratio as it exists • When dead, no more 14C incorporated and that present begins to decay • Assign a date to the object based on how much 14C is left • Allotropes • Diamond is the densest form • Tetrahedral single bonds at all C • Hardest of all natural substance • Graphite is the thermodynamically most stable form • Layers of fused 6-membered rings with all C sp2 hybridized • Pi-bonding between layers allows lubrication and electrical conduction 5730 year half-life

3. Carbon Allotropes

4. Fullerenes = spherical or tubular C oligomers of 5- and 6 membered rings • Buckminsterfullerene = C60 discovered in 1985 (Nobel Prize) • Found in nature at meteorite impact sites • C70, C80, Carbon Nanotubes prepared soon after by Laser vaporization of graphite: Kroto, Curl, Smalley 1996 Nobel Prize in Chemistry • Easily functionalized and used to trap small molecules • Hard to synthesize in large amounts • Non-fullerene –C—C≡C—C≡C—C≡C— also identified in nature • Si and Ge also have diamond structures, but much weaker bonds Ge(mp) = 945 oC while C(mp) = 4100 oC • Compounds • “Inorganic” Carbon Compounds a) Carbon can be 5- or 6-coordinate (or even more) when bridging metals Crystalline C60

6. Carbon Monoxide :C≡O: • Stable carbon with only 3 bonds • Toxic gas byproduct of combustion; takes place of O2 in hemoglobin • Carbon Dioxide O=C=O • Component of the atmosphere; product of respiration and combustion • Greenhouse effect = reabsorption of heat bouncing off Earth • Carbon dioxide concentrations increasing since industrial revolution • Global Climate change is predicted • Carbides = anionic carbon ions or clusters • C4- is simply called Carbide • C22- is called acetylide • C34- is called tricarbide • Reactive ions Al4C3 + 12 H2O ----> 4 Al(OH)3 + 3 CH4 CaC2 + 2 H2O ----> Ca(OH)2 + HC≡CH (exothermic, flame!) • Si 2nd most abundant (27%) on Earth’s crust (O = most abundant) a) Semiconductors and Silicates (chapter 7)

7. Silanes = molecules with Si—Si bonds • Unstable because Si—Si (340 kJ/mol) weaker than C—C (368 kJ/mol) • Si less electronegative than H (1.90 vs. 2.20) so attacked by Nu • Si larger than C, which also makes it more easily attacked by Nu • Empty d-orbitals act as e-pair acceptors • Ge, Sn, Pb all exhibit the Inert Pair Effect: PbX2, SnX2, etc… II. Group VA

8. The Elements • As, Sb (metalloids) and Bi (metal) known since antiquity • Variety of allotropes for all of them • Bi is the heaviest nonradioactive element • P is a nonmetal first isolated from urine by Brandt in 1669 • Glowed when exposed to air: “light bringing” = name • Has many solid allotropes • White P4 has cubic and hexagonal forms that react with air--store in H2O • Red P has an amorphous structure and can be formed by heating white P • Black P has 6-membered rings, converts to other forms at high Pressure • P4 tetrahedron in liquid and gas phases black red white black white

9. Nitrogen is a nonmetal diatomic gas making up 78.1% of Earths’ atmosphere • Isolated from air in 1772 by Rutherford, Cavendish, and Scheele • Colorless, odorless, nonreactive gas • :N≡N: structure has strong stable triple bond • Used as inert atmosphere for performing chemical reactions • N2(l) (bp = 77K) useful for cooling and trapping other compounds • Ions • N3- = azide = useful inorganic anion (like Cl- or Br-) • N3- = nitride = strong ligand in some metal complexes (gives 6 e-) • N22- = “pernitride” = long bond (122.4pm) compared to N2 (109.8pm) • N5+ unstable cation prepared as [N5+][AsF6-] in 1999 • Compounds • Hydrides • Ammonia = NH3 • Used as a fertilizer and in chemical synthesis and industry • Nitrogen Fixation = converting N2 into NH3 • Done under mild conditions by bacterial enzyme • Done at 300 oC and 200 atm by Haber-Bosch Industrial Process • Natural gas and energy expensive, but no better alternative yet N2 + 3H2 + catalyst ----> 2 NH3

10. Hydrazine = N2H4 used as rocket fuel N2H4 + O2 ----> N2 + 2 H2O DH = -622 kJ/mol • Phosphine = PH3 = poisonous gas • Weaker intermolecular attraction than NH3 (little H-bonding) • mp, bp < NH3 • Phosphine derivatives (PR3) important Lewis Base Ligands • Nitrogen Oxides and Oxyions • Nitrous Oxide = N2O = dental anesthetic • Decomposes to N2 and O2 (nontoxic) • Suspected greenhouse gas • Nitric Oxide = NO • Coordinating Ligand forms metal complexes • Biological functions (Viagra helps regulate it) • N2O4 and NO2 Equilibrium • N2O4(g) 2 NO2(g) DH = 57.3 kJ/mol • N2O4 = colorless and diamagnetic • NO2 = brown and paramagnetic

11. NOx Pollution • NO formed from atmospheric N2, O2 during fossil fuel combustion • Can be oxidized to NO2; collectively they are known as NOx • Acid Rain: 3 NO2 + H2O ----> 2 HNO3 + NO • Nitric Acid = HNO3 is an Industrially Important Acid • Used in the synthesis of ammonium nitrate fertilizer (much N content) • Ammonium Nitrate is also an explosive 2 NH4NO3 ----> 2 N2 + O2 + 4 H2O • Synthesis of nitric acid 4 NH3 + 5 O2 ----> 4 NO + 6H2O 2 NO + O2 ----> 2 NO2 3 NO2 + H2O ----> 2 HNO3 + NO • Phosphoric Acid = H3PO4 is also Industrially Important P4 + 5 O2 ----> P4O10 P4O10 + 6 H2O ----> 4 H3PO4 Ca3(PO4)2 + 3 H2SO4 ----> 2 H3PO4 + 3 CaSO4

12. Group VIA Chalcogens • The Elements • Se = nonmetal important to the xerography industry as a photoconductor • Te = metalloid not important industrially • Po = radioactive metal discovered by Curie (1898) and used to power satellites • O = nonmetal diatomic gas making up 21% of Earth’s atmosphere, 46% crust • Discovered in 1770’s by Priestly • O2 is paramagnetic: p2p* __ __ • Important in oxidation reactions • Respiration by binding to Hemoglobin • O3 form = Ozone shields Earth from UV = 200-360 nm • O3 is formed in the atmosphere form O2 O2 + hn ----> 2 O O + O2 ----> O3 • UV Protection: 2 O3 + hn ----> 3 O2 • Ozone Depletion NO2 + O3 ----> NO3 + O2 CF2Cl2 + hn ----> CF2Cl + Cl NO3 ----> NO + O2 Cl + O3 ----> ClO + O2 NO + O3 ----> NO2 + O2 ClO + O ----> Cl + O2 2 O3 ----> 3 O2 O3 + O ----> 2 O2

13. Liquid oxygen has a blue color due to absorption of light • Diamagnetic, red, O8 allotrope discovered 2006—unpaired O2 e- form bonding MO’s Excited states Ground state

14. S = yellow solid nonmetal • Known biblically as Brimstone • Makes gunpowder with KNO3 and charcoal • Many allotropes: a-S8 orthorhombic is most stable • Viscosity of solid varies unexpectedly with temperature • Sulfuric Acid is the #1 Industrial Chemical (43.25 x 109 kg/yr in US) • Dense (d = 1.83 g/cm3), viscous, colorless liquid that burns skin • Exothermic reaction with water (chars sugar by removing H2O) • Synthesis: S + O2 ----> SO2 or MxSy + O2 ----> ySO2 + MxOy SO2 + ½ O2 ----> SO3 SO3 + H2O ----> H2SO4 ring----------->S8 chain---->S16 chain---->S24 chain

15. F2 Br2 Cl2 I2 • Group VIIA Halogens • The Elements • Salts known since antiquity as food preservatives (name = “salt former”) • Discovery as elements • Cl2 in 1630 by van Helmont (HCl known since ~900); toxic yellow gas (bleach smell) • I2 in 1811 by Courtois; purple solid that easily sublimes • Br2 in 1826 by Balard; red-brown liquid that easily vaporizes • F2 in 1886 by Moissan; so reactive it is hard to even handle • At2 in 1940 during particle collisions (radioactive with ½ life = only 8 hours) • Chemistry dominated by ready reduction to X- • Excellent oxidizing agents (is reduced itself); F2 is best of all elements • F is the most electronegative element, decreases down the group • I2 has the highest London Forces, which is why it is solid • Fluorine Discrepancies • Low bond energy in F2 makes it more reactive than expected • Extrapolate from others to 290 kJ/mol; actually 159 kJ/mol • Repulsion of lone pairs responsible; also has long 143 vs. 128 pm bond • MO explanation: poor overlap of small orbitals b) HF weak acid: stronger H—F bonds not easily dissociated

16. Polyatomic Ions • I2 + I- ----> I3- + other polyiodides • I2 + oxidizing agent ----> I2+ + I42+ • Interhalogens: multiple possibilities and ways to synthesize them • Cl2 + F2 ----> 2 ClF • I2 + AgF ----> IF + AgI 7) Pseudohalogens = dimeric compounds with properties similar to X2

17. Group VIIIA Noble Gases • The Elements • Discovery in 1766 by Cavendish • Nonreactive, leftover fraction of air (after N2, O2, CO2 all reacted) • Not separated from each other • He discovered in 1868 by Locklear and Frankland • New emission line observed during a solar eclipse • Helios = Greek for “sun” • Rare on Earth (Dexter, Kansas); used as NMR coolant (bp = 4.2 K) • Second most abundant in Universe ( H = 76%, He = 23%) • Ar discovered in 1890 by Rayleigh and Ramsay • N2 density discrepancies must be accounted for by a contaminant • Argon = “no work” because does not react • Used as chemically inert atmosphere, and for filling light bulbs

18. Ne, Kr, Xe discovered 1890’s by Ramsay and Travers • “Neon Lights” due to emission from gases when electricity passes through • “Hyperpolarized” 129Xe used as MRI contrast agent in lungs • Rn isolated from nuclear decay products in 1902 • Product of Uranium decay in minerals • Enters homes through basements and floors • Radioactive and causes lung cancer (gas is inhaled) • Chemistry of Noble Gases • Thought to be inert because of filled valence shells • Clathrates = noble gas trapped in a cage of other molecules • Crystallized solvents under pressure; gas in cavities (1940’s) • He, Ne too small and never trapped yet • Bartlett, 1962 makes first true Noble Gas compounds • Xe + PtF6 ----> Xe+[PtF6]- • XeF2, XeF4 made soon after • Only Kr, Xe, Rn have been made into compounds so far, usually with F,O,Cl • Reactivity of Compounds • Fluoronating agents: 2 SF4 + XeF4 ----> 2SF6 + Xe • Oxidizing agents: XeO3 + 6 H+ + 6 e- ----> Xe + 3 H2O Eo = +2.10 V