Concept 09 / 09The p-Block Elements
Group 18: Noble Gases and Compounds of Xenon
Electronic configuration, ionisation enthalpy and physical properties of noble gases, Bartlett's discovery of noble gas compounds, preparation, structures (VSEPR) and hydrolysis of xenon fluorides and oxides, and uses of helium, neon, argon and xenon.
Group 18: helium, neon, argon, krypton, xenon, radon. Configuration (He: ), a closed shell.
Properties. Very high ionisation enthalpies (decreasing down the group), large positive electron gain enthalpies, monatomic gases held only by weak dispersion forces. Boiling points are very low and increase down the group with polarisability (He , the lowest of any substance). Solubility in water also increases down the group. Helium diffuses through most materials and cannot be solidified at ordinary pressure.
Discovery of compounds. In 1962 Neil Bartlett prepared by oxidising with . Noticing that the first ionisation enthalpy of () is almost the same as that of xenon (), he made the first noble gas compound, a red solid formulated as . Xenon, with the lowest ionisation enthalpy among the stable noble gases, forms the most compounds (with F and O); krypton forms ; He, Ne and Ar form no stable compounds under ordinary conditions.
Xenon fluorides. Made by direct reaction, the product depending on the Xe : ratio:
(xenon in excess, , );
(ratio , , );
(ratio , , to ); also .
They are colourless crystalline solids and powerful fluorinating agents.
Structures (VSEPR, see figure): linear (, three equatorial lone pairs); square planar (, two trans lone pairs); distorted octahedral (, one lone pair).
Fluoride donor and acceptor behaviour: ; (M = Na, K, Rb, Cs).
Hydrolysis and oxides.
.
(disproportionation).
(complete).
Partial hydrolysis of : ; .
is a colourless explosive solid with a pyramidal shape (one lone pair); is square pyramidal; has a see-saw shape.
Uses. Helium: filling balloons and airships (light and non-inflammable), mixed with oxygen for deep-sea diving (much less soluble in blood than nitrogen, preventing the bends), cryogenics and superconducting magnets (MRI). Neon: discharge signs (orange-red glow), beacon lights. Argon: inert atmosphere for welding and metallurgy, filling light bulbs. Xenon and krypton: high-intensity lamps and photographic flash tubes. Radon: radiotherapy (radioactive).
Key Takeaways:
- Closed shells: high IE, positive electron gain enthalpy, monatomic, weak dispersion forces.
- Bartlett: and Xe have almost equal IE, leading to .
- linear, square planar, distorted octahedral, pyramidal, square pyramidal.
- Complete hydrolysis of gives and HF.
- Pitfall: hydrolysis is a disproportionation that also releases Xe and .
Brain-Teaser 1: Find the number of lone pairs on Xe and the shape of .
Solution: Xe has valence electrons; it forms one Xe=O (using ) and four Xe-F (using ), leaving electrons, one lone pair. Steric number ( bonded atoms plus lone pair): square pyramidal, with O at the apex.
Brain-Teaser 2: How many moles of HF form when of is completely hydrolysed, and what is the oxidation state of Xe in the other product?
Solution: : HF. Xenon stays at in (this hydrolysis is not a redox reaction).
Brain-Teaser 3: Why does helium not form compounds while xenon does?
Solution: Helium's electrons are extremely tightly held (IE ) and it has no low-lying empty orbitals to use in bonding. Xenon's valence electrons are far from the nucleus and well shielded (IE ), so the very electronegative F and O can oxidise it and form bonds, with xenon expanding its valence shell.
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