Ch 18. Group 18
2
Elements
1785 Henry Cavendish air contains < 1% of gas that does not react with O2 under electric discharge
1868 (Aug 18) Janssen / Lockyer solar eclipse, yellow line in solar spectrum (He)
1895 Lord Raleigh N2 in air is denser than N2 from ammonia (due to Ar in air sample)
Commercial sources: no naturally occurring compounds
He – impurity in natural gas (from radioactive decay underground)
Ar to Xe – distillation of liquid air
Rn – none, intensely radioactive (t1/2 ~ 4 days)
3
Applications
He coolant (bp = 4.2K, lowest known); esp. for superconductors (MRI/NMR)
balloons, etc. lifting power = 1 kg / m3
diving air tanks, He has a lower blood solubility than N2
Other gases
inert atmosphere (ex – welding)
lighting (electric discharge to get excited state)
4
Noble gas chemistry I / eV
He 24.6
Ne 21.6
Ar 15.8
Kr 14.0
Xe 12.1
Rn 10.7
5
Compounds
Consider MO theory – bonding depends on symmetry and orbital energies, trends throughout the p-block
Xe (g) + F2 (g) XeF2 (use excess Xe)
Xe + 2 F2 XeF4 (D4h)
Xe + 3 F2 XeF6 (use excess F2)
XeF2 colorless sublimable solid
linear molecular structure, isoelectronic w IF2-,
Xe-F b.l. ~ 200 pm
400 C or uv
6
XeF2, XeF4, and XeF6
XeF6 or (XeF5+F-)n
XeF4 crystals
7
XeFn reactions
XeF2 + SbF5 XeF+SbF6 (s)
+ HN(SO2F)2 XeF+N(SO2F)2 (s) + HF
+ HSO3F XeF+SO3F (s)
XeF6 + SbF5 XeF5+SbF6
(s)
XeF6 + 2 MF M2XeF8 square anti-prism
(Xe-F)+ isoelectronic w IF, Xe-F b.l. ~ 195 pm
8
XeF+ and XeF5+
< FeqXeFeq = 87°
< FaxXeFeq = 78°
XeF5+RuF6
- (s) XeF+SO3F-
XeF+N(SO2F)2-
9
Xenon oxidesXeF2 + H2O Xe + 2 HF + ½ O2
XeF6 + H2O XeO3 (aq) + 6 HF
(metathesis instead of hydrolysis is possible under controlled conditions)
XeO3 + OH HXeO4
disproportionates in base
XeO64 + Xe
XeO3 is endoergic,and explosive
(perxenate)
10
Xenon oxyfluorides
also: XeO2F2 , XeOF4, etc
11
Other compounds
AuF3 [AuXe4](Sb2F11)2
Xe(g)
SbF5/HF(superacid)
12
Krypton chemistry
Kr + F2 KrF2(s)
unstable at RT, Hf ~ + 20 kJ/mol
KrF2 + SbF5 (KrF)+ SbF6-
a few other LA complexes are known
h
-78°C