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Ch 8 Main Group Elements. General Trends Introduction Twenty top industrial chemicals are main group elements or compounds 8 of the top 10 are “inorganic” 9 of the top 20 are “organic” Provide context for concepts also important later Clear up some misconceptions C with > 4 bonds

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ch 8 main group elements
Ch 8 Main Group Elements
  • General Trends
    • Introduction
      • Twenty top industrial chemicals are main group elements or compounds
        • 8 of the top 10 are “inorganic”
        • 9 of the top 20 are “organic”
      • Provide context for concepts also important later
      • Clear up some misconceptions
        • C with > 4 bonds
        • Alkali metal anions
        • Noble Gas compounds
Physical Properties
    • Main group elements characterized by s/p electrons following the octet rule
      • Metals give up electrons to achieve filled valence shell
      • Nonmetals accept electrons to achieve filled valence shell
    • Conductance
      • Metals have loosely bound

e- so they can conduct electricity

Electron Sea Model

b) Nonmetals have localized

lone pairs = insulators

c) Metalloids/Semimetals

        • Diagonal from B to Po
        • Can often be intrinsic or

doped semiconductors


3) Electronegativity

a) F most electronegative; trend is to decrease down and to left in periodic table

b) H has different value than other Group I elements: duet rule

c) Noble Gases calculated to be more electronegative than halogens

i. Smaller size due to larger Z

ii. Should pull on shared e- quite strongly

Ionization Energy
    • E ----> E+ + e-
    • Trends similar to electronegativity with a few exceptions
      • B < Be because Be has filled subshell 2s2 and B easily loses 2s22p1
      • O < N because N has half-filled subshell 2s2p3 and O easily loses 2s22p4
Chemical Properties
        • Elements within a group have similar reactivities because of same valence
        • Diagonal (upper left to lower right) have some similarities
          • Electronegativity about the same: B-Si-As-Te all between 1.9-2.2
          • Solubility: LiF, MgF2 sparingly soluble
          • Size and electronic structure probably most important
        • First Row Anomaly for Li through Ne
          • Significant differences from rest of their groups
          • F2 much weaker bond than extrapolated from I2, Br2, and Cl2
          • HF weak acid while HI, HBr, and HCl are strong acids
          • C=C, C≡C prevalent, while Si=Si and Si≡Si very rare
          • Strong H-bonding for all first row elements
          • Result of small size and high Electronegativity
  • Hydrogen
    • Position in Periodic Table
      • 1s1 electron configuration is like Alkali Metals but not chemically similar
      • One e- short of octet is like Halogens, but has limited similarity

H2 like X2 H- forms like X-

What about placing H in Group IVA?
      • Half-filled valence shell
      • Electronegativity is about the same
      • Covalent bond formation favored over ionic
    • Best to Treat H as unique, not part of any other Group
  • Abundance
    • Most abundant element in the universe; found mostly in stars and in “space”
    • 3rd most abundant element on Earth’s Surface; found mostly in compounds (H2O)
    • Isotopes
      • Hydrogen = 1H = 99.9844%
      • Deuterium = 2H = D = used in NMR solvents and “isotope effect” studies
      • Tritium = 3H = T
        • Radioactive with 12.35 y half-life:
        • Produced in Nuclear Reactors:
        • Used as tracer for H absorption; ground water flow detection
Chemical Properties
    • Gains e- to achieve Noble Gas configuration = Hydride = H:-
      • Ionic compounds analogous to halides: NaH, KH, etc...
      • Covalent C—H bonds (consider 2 e- in bond as making H into a hydride)
      • Lone pair donor (Lewis Base) functioning as Ligand to metals: Li2[NiH4]
      • Organic Reducing agent: NaBH4, LiAlH4
    • Loses one e- to become H+ = Proton
      • Always associated with other molecules due to small size: H3O+
      • Basis for one definition of Acids/Bases
      • Can form Hydrogen Bonds
    • Combustion
      • H2 + ½ O2 ----> H2O
      • Pollutant free energy source?
      • Problems:
        • Generating cheap H2
        • Currently, most H2 comes from natural gas (generates CO2)
        • Ideally, need cheap way to generate from water
        • Storage is also a problem: metals? carbon nanotubes?
Group IA Alkali Metals
    • The Elements
      • Known since antiquity in salts (NaCl) used for food flavoring and preservation
      • K, Na essential for human life
      • Pure elements discovered “recently” due to difficulty in reduction of cations
        • 1807 Davy discovered K, Na from electrolysis of molten KOH, NaOH
        • 1817 Davy discovered Li from Li2O
        • Cs, Rb discovered 1860, 1861 using newly invented spectroscope
        • Fr discovered in 1939 as a radioactive isotope from the decay of Actinium
      • Physical Properties
        • Silvery metals that are highly reactive and have low mp
        • Stored under oil to prevent reaction with water in air
        • Soft enough to cut with a knife by hand
Chemical Properties
    • Easy loss of 1 e- (lowest IE of all the elements)
      • Excellent reducing agents (get oxidized themselves)
      • 2 Na + 2 H2O ----> 2 NaOH + H2 Exothermic
    • Can form oxides (M2O), peroxides (M2O2), and superoxides (MO2)
    • Dissolve in NH3(l) to give solvated e-
      • Na + x NH3 ----> Na+ + e(NH3)x-
      • Even better conductor than salt water, about as good as a molten metal
      • Paramagnetic because of unpaired e-
      • Less dense than NH3(l) because e- forms cavities in solution
      • Blue color at low conc. (e-); coppery color at high conc. (M-)
      • Excellent reducing agents
      • Slowly decompose to form MNH2 + ½ H2
    • M+ forms Lewis Acid/Lewis Base Complexes
      • Crown Ethers = cyclic –[CH2CH2O]- polyethers donate lone pairs from O
      • Size selective for specific alkali metal cation
      • HSAB argument: hard O and hard alkali metal cation

Crown Ether,

Cryptand and


Formation Constants

of Cryptands with

Alkali Metal Cations

Alkalides = M- (James Dye)
        • 2 Na + [2.2.2]Cryptand ----> Na([2.2.2]Cryptand)+ + Na-
        • Disproportionation: 2 Na ----> Na+ + Na-
        • Alkalides have an s2d10 electron configuration
        • Very powerful reducing agents
        • Usually unstable and decompose quickly even at room temperature
  • Group IIA Alkaline Earths
    • The Elements
      • Ca and Mg are abundant and have been known since antiquity
        • CaO = lime used by Romans in brick mortars
        • CaSO4• 2 H2O = gypsum used by Egyptians in plasters
        • Mg is strong, light construction material
        • Ca found in bone, teeth; other biological functions
      • Sr and Ba less abundant, but also found as Sulfates and Carbonates
      • Be even less abundant, found in Beryl = Be3Al2(SiO3)6 = emerald, aquamarine
        • used in alloys with Cu, Ni, etc…
        • Reduces corrosion
      • Ra is radioactive, discovered in 1898 by Marie Curie
Smaller than Group IA elements due to larger Z
      • More dense
      • Higher I.E.
      • Higher mp, bp, DHfus, DHvap
  • Chemical Properties
    • Tend to lose 2 e- and are thus good reducing agents
      • Mg + 2 H+ ----> Mg2+ + H2
      • More reactive down group: Ca + 2 H2O ----> Ca(OH)2 + H2
    • Be is unique: primarily covalent bonds rather than ionic; also extremely toxic
Grignard Reagents = RMgX
        • Complex structures
        • RMgX + H2C=O ----> RCH2OH
      • Photosynthesis depends on Mg
        • Chlorophyll contains Mg in a macrocycle
        • Complex chain of reactions to produce sugars from light, CO2
      • Cement = complex mixture of Ca silicates, aluminates, and ferrates
        • Most important construction material (1021 kg/yr worldwide)
        • With H2O and sand, concrete is formed
        • H2O and OH- link the other components into large, strong crystals
  • Group IIIA
    • Boron
      • Nonmetal chemically most similar to C and Si
        • Hydride formation like C
        • Borates = Oxygen containing minerals like Silicates
      • Pure element has many allotropes, usually containing B12 icosahedral units
Boranes = BxHy compounds
    • H often bridges B atoms in boranes
    • Diborane = B2H6
      • 12 valence electrons, 8 involved in terminal B—H bonds
      • 4 electrons in two 3-center, 2-electron bonds B—H—B
      • Group Theory analysis of D2h symmetry produces Ag and B1u bonding Molecular Orbitals
Carboranes = CxByHz compounds also have bridging alkyl groups
    • Bridging also seen in Al compounds
  • Isotopes
    • 11B (80.4%) and 10B (19.6%)
    • 10B high neutron absorption (M.F. Hawthorne)
      • Used in cancer therapy
      • B compounds localize in cancer cells
      • Irradiate with neutrons
      • Kill cancer cell specifically
Metallic Group IIIA Elements
    • Formation of 3-coordinate Lewis Acids
      • BX3, AlX3 excellent Lewis Acids: AlX3 + RX ----> R+ + AlX4-
      • Accept lone pair form Lewis Base to fill octet and become 4-coordinate
    • Usually form 3+ cations
    • Inert Pair Effect = metal with oxidation state 2 less than valence number
      • Tl ----> Tl+ + e- (Also seen in other metals: Pb ----> Pb2+ + 2 e-)
      • Retains s electrons because entirely filled subshell is somewhat stabilized
      • Explanation is actually more complex
Parallels to Organic Chemistry
    • Borazine = inorganic benzene = B3N3H6
    • Chemistry is different even though structure is similar
      • Polar B—N bond
      • Nu attack N; E attack B more easily than in Benzene
    • BN = boron nitride
      • Diamond-like structure is hard like diamond
      • Graphite Structure is poor conductor because less delocalized electrons