Download Chapter 8. Chemistry of Main Group Elements

Chapter 8. Chemistry of Main Group Elements
▪ In this chapter: most significant physical and chemical data on each of the main group elements
→ “representative elements”
Group 1,2 and 13-18
8.1 Group Trends in Main Group Chemistry
8.1.1 Physical Properties
- e- configuration: total # of e- in the outmost shells =
American group #
in the periodic table
last digit if group # by IUPAC
- metal, semimetal and nonmetal:
far left: alkali metal
alkaline earth metals
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luster, conduct heat & electricity
malleability
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Chapter 8. Chemistry of Main Group Elements
Fig.8.1 metals vs. nonmetals via difference in electrical resistivity
left
right
alkalimetals
nonmetals
resistivity ↓
resistivity ↑
(conductivity ↑)
(conductivity ↓)
loosely bonded ve-
localized ve-
ve- free to move
less mobile
∴ conduct current
∴ nonconductive
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Chapter 8. Chemistry of Main Group Elements
- metalloid/semimetals: along a diagonal from B to Po
: intermediate b/w metallic and nonmetallic
: Si, Ge
finely tuned conductivity by impurities → important in semiconductor
industry
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Chapter 8. Chemistry of Main Group Elements
8.1.2 Electronegativity
- Fig. 8.2: a guide to the chemical behavior
: extremely high EN for F, He, Ne
steady decline toward the left & the bottom of the PT
: semimetals → intermediate EN
8.1.3 Ionization Energy
- Fig. 8.3: similar pattern to EN with some subtle differences
: general increase toward upper right-hand corner
: but, some exceptions →
Be
vs.
2s2
2s22p1
(complete)
N
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B
(+1 e-)
vs.
O
2p3
2p4
(complete)
(+1e-)
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Chapter 8. Chemistry of Main Group Elements
8.1.4 Chemical Properties
- strong similarities: within each group
: along diagonals
e.g) EN: from B to Te (1.9 – 2.2)
low solubility: LiF & MgF2
complex 3D structures of SiO4, BO4
8.3 Group 1: The Alkali Metals
8.3.1 The Elements
- Silvery (except Cs – golden appearance)
- highly reactive, low-m.p.
- stored under nonreactive oil – to prevent oxidation
- soft enough to be cut with a knife
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Chapter 8. Chemistry of Main Group Elements
8.3.2 Chemical Properties
- Table 8.3
- ease to lose one e- → lowest ionization E
to achieve noble gas config.
- excellent reducing agents
- highly reactive with water → to produce H2
2Na + 2H2O → 2NaOH + H2
highly exothermic, could be explosive due to H2
should be prevented from reacting with water
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Chapter 8. Chemistry of Main Group Elements
- reactive with O → form oxides, peroxides, superoxides
- Dissolved in ligand ammonia or other donor solvent
produce →solvated eNa + xNH3 → Na+ + e(NH3)dilute solutions of alkali metals in ammonia → conduct electricity
at high-concentration: bronze metallic luster
: comparable conductivity to molten metals
paramagnetic: one solvated e- per metal atom
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Chapter 8. Chemistry of Main Group Elements
good reducing agents: e.g. RC≡CH + e- → RC≡C- + 1/2H2
NH4+ + e- → NH3 + 1/2H2
S8 + 2e- → S82unstable, undergo slow decomposition to form amide
M + NH3 → MNH2 + ½ H2
- forming complexes with a variety of Lewis bases (ligands)
cyclic Lewis bases
e.g. Fig.8.6: (1) “crown” ethers
donate e- to metals
(2) cryptands: 8 donor atoms
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Chapter 8. Chemistry of Main Group Elements
(2) cryptands: size match b/w metal ion & cage is critical !!
Fig. 8.7: e.g.
Cs+ → C322
Li+ → C211
(3) metallacrowns: incorporates metals into the crown str.
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Chapter 8. Chemistry of Main Group Elements
- alkalides (alkali metal anions)
e.g. (1) Na- : 2Na + N{(C2H4O)2C2H4}3N → [NaN{C2H4O}2C2H4}3N]+ + Nacryptand [2,2,2]
[Na(cryptand [2,2,2])]+
- Fig. 8.8: (a) crown ether sandwich electride: Cs+(15C5)2e(b) Na-H+36
(c) thermally stable electride
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Chapter 8. Chemistry of Main Group Elements
8.4 Group 2: The Alkaline Earths
8.4.1 The Elements
- Table 8.4
- Ca, Mg: most abundant elements in the Earth’s crust
Ca(5th), Mg(6th)
- Ra: radio active isolated by Pierre & Marie Curie
- smaller than Group1 due to the greater nuclear charge
- more dense& have higher IE
- higher m.p. & b.p.
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Chapter 8. Chemistry of Main Group Elements
- Table 8.4
- applications: Mg alloy – strong, but very light construction materials
- ¼ density of steel
- Ra: cancer treatment
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Chapter 8. Chemistry of Main Group Elements
8.4.2 chemical Properties
- generally, tend to loose two e- → noble gas electron config.
good reducing agent
- react with acid
Mg + 2H+ → Mg2+ + H2
- react with water (Ca and heavier)
Ca + 2H2O → Ca(OH)2 + H2
- Be: BeX2 has various form depend on the condition
: Fig. 8.9
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Chapter 8. Chemistry of Main Group Elements
8.5 Group 13
8.5.1 The Elements
- Table 8.5
- Boron: pure element → icosahedral B12 unit
: so different from the other members of this group
: more similar to C → form hydrides
Si → oxygen-containing complex str. (borates)
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Chapter 8. Chemistry of Main Group Elements
- Borane (boron hydride, B2H6): contains bridging H
total 12 valence electrons → 8 e- for bonding to the terminal H
→ 4 e- for bonding in the bridge
three-center, two-electron bonding
to understand,, use group orbital approach with D2h (Chapter 5)
Fig.8.11: boron group orbital + bridging H group orbitals
match the labels of irred. Rep.
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Chapter 8. Chemistry of Main Group Elements
- Fig.8.11: boron group orbital + bridging H group orbitals
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Chapter 8. Chemistry of Main Group Elements
- Fig.8.12: B3u symmetry → one-bonding & one-antibonding
major component for the
stability of the bridge
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Chapter 8. Chemistry of Main Group Elements
- Fig.8.13: two more bonding with Ag
total three bonding orbitals for the bridging
shape of these orbitals
Fig.8.14
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Chapter 8. Chemistry of Main Group Elements
- Fig.8.15: other bridging components Borane, Carboranes, Bridged Al compounds
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Chapter 8. Chemistry of Main Group Elements
8.5. 2 Other Chemistry of the Group 13 Elements
- metallic nature increases going down the group
- usually form 3+ ions
- Inert pair effect: Th → 1+ ion as well as 3+ ion
two less oxidation state than American group #
*Pb → 2+ ion as well as 4+ ion
reason: entirely filled subshell (s2 subshell “ inert pair)
- parallels b/w main group & organic chem.
borazine (B3N3H6) vs. benzene
difference: polarity of B & N
more susceptible to attack
by nucleophiles
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