General Trends

1
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 gt 4 bonds
• Alkali metal anions
• Noble Gas compounds

2

• 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
• b) Nonmetals have localized
• lone pairs insulators
• c) Metalloids/Semimetals
• Diagonal from B to Po
• Can often be intrinsic or
• doped semiconductors

Resistivity
3

• 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

4

• Ionization Energy
• E ? E e-
• Trends similar to electronegativity with a few
  exceptions
• B lt Be because Be has filled subshell 2s2 and B
  easily loses 2s22p1
• O lt N because N has half-filled subshell 2s2p3
  and O easily loses 2s22p4

5

• 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
• CC, CC prevalent, while SiSi and SiSi 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

6

• 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 Earths 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

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• Chemical Properties
• Gains e- to achieve Noble Gas configuration
  Hydride H-
• Ionic compounds analgous to halides NaH, KH,
  etc...
• Covalent CH bonds (consider 2 e- in bond as
  making H into a hydride)
• Lone pair donor (Lewis Base) functioning as
  Ligand to metals Li2NiH4
• 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

8

• 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

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• 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

10
Crown Ether, Cryptand and Complex
Formation Constants of Cryptands with Alkali
Metal Cations
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• Alkalides M-
• 2 Na 2.2.2Cryptand ? Na(2.2.2Cryptand)
  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

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• 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

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• Grignard Reagents RMgX
• Complex structures
• RMgX H2CO ----gt 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

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• Boranes BxHy compounds
• H often bridges B atoms in boranes
• Diborane B2H6
• 12 valence electrons, 8 involved in terminal BH
  bonds
• 4 electrons in two 3-center, 2-electron bonds
  BHB
• Group Theory analysis of D2h symmetry produces Ag
  and B1u bonding Molecular Orbitals

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• 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
• Used in cancer therapy
• B compounds localize in cancer cells
• Irradiate with neutrons
• Kill cancer cell specifically

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• 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
• Th ? Th 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

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• Parallels to Organic Chemistry
• Borazine inorganic benzene B3N3H6
• Chemistry is different even though structure is
  similar
• Polar BN bond
• Nu attack B E attack N more easily than in
  Benzene
• BN boron nitride
• Diamond-like structure is hard like diamond
• Graphite Structure is poor conductor because less
  delocalized electrons