Noble Gases (Group 18)

Why noble gases matter

• Bartlett 1962 broke the inertness dogma. Neil Bartlett at UBC observed that PtF₆ oxidizes O₂; he noticed Xe's ionization energy (1170 kJ/mol) is essentially the same as O₂'s (1175 kJ/mol) and predicted Xe would also be oxidized. The orange-yellow XePtF₆ he made was the first noble-gas compound. Within a year XeF₂, XeF₄, XeF₆, and KrF₂ followed.
• Argon makes industrial welding possible. ~0.93% of Earth's atmosphere, harvested at ~70 Mt/yr by cryogenic distillation. Inert atmosphere for TIG welding stainless and aluminum, blanket gas for chemical reactors, and fill gas for incandescent and fluorescent lamps. Cheaper than nitrogen for purposes where Ti, Mg, or Li would react with N₂.
• Helium is needed at scale and is non-renewable on human timescales. Cryogenic He cools NMR magnets, MRI scanners (4.2 K), and superconducting accelerator magnets. The US Federal Helium Reserve at Cliffside, Texas, accumulated over 1 Bcf in the 20th century; supply now depends on natural-gas wells in Algeria, Qatar, and Russia. Atmospheric He is only 5.2 ppm — extraction from air is uneconomic.
• Xenon anesthesia. At ~70% partial pressure Xe is a general anesthetic with hemodynamic stability superior to nitrous oxide or isoflurane. Approved in Russia and parts of Europe; cost (Xe is 0.087 ppm of air) is the chief barrier. Krypton and argon also show narcotic effects at elevated pressure (relevant for deep-sea diving).
• Excimer lasers use noble-gas halides. KrF (248 nm), ArF (193 nm), and XeCl (308 nm) excimer lasers are foundational to deep-UV photolithography that prints the smallest features in modern semiconductors. ArF immersion lithography drove sub-90 nm CMOS through ~14 nm before EUV took over.
• Plasma displays and lighting. Krypton in plasma displays (KrCl excimer); xenon in HID automotive headlights; neon in classic neon signs (orange-red discharge); argon plus mercury in fluorescent tubes. Each color is the discharge spectrum of an excited noble-gas atom returning to ground.
• Radon is a major lung-cancer risk. ~21,000 lung-cancer deaths/yr in the US (per EPA) attributable to radon, the second-leading cause after smoking. Radon-222 (half-life 3.82 days) diffuses from soil and concentrates in basements; sub-slab ventilation is the standard mitigation.

Common misconceptions

• All noble gases are completely inert. Disproved 1962. XeF₂, XeF₄, XeF₆, XeO₃, XeO₄, KrF₂, RnF₂, and matrix-isolated HArF and HKrF are all known. Even helium forms transient HeH⁺ (the first molecular ion to form in the early universe, detected in space 2019).
• The octet rule is universal. Helium achieves stability with 2 electrons (duet), not 8. Hydrogen also takes only 2. Beyond Period 3, expanded octets are common (PCl₅, SF₆, IF₇); the octet is a Period-2 phenomenon, not a universal law.
• Argon is a trace gas. Argon is the third most abundant atmospheric gas at 0.934% by volume — more abundant than CO₂. Most Ar-40 originates from K-40 beta-decay in Earth's crust over billions of years, which is why atmospheric Ar is overwhelmingly Ar-40 rather than Ar-36.
• Helium-4 from the air. The atmosphere has 5.2 ppm He, but cryogenic distillation of air to that ppm level is uneconomic. Industrial He comes from natural gas wells where alpha decay has accumulated He-4 over geological time at 0.3-7% concentrations.
• Noble-gas compounds are stable at room temperature. XeF₂ is a stable white solid; XeF₄ too. But KrF₂ decomposes above -45 °C, and most argon compounds (HArF) require cryogenic matrices below 17 K. Stability decreases sharply going up the group as ionization energies rise.
• Oganesson (Z = 118) is just heavier xenon. Relativistic calculations predict Og will not behave as a typical noble gas — relativistic contraction of 7s and 7p_1/2 shells suggests a partly metallic character. Only a handful of Og atoms have been synthesized; the longest-lived isotope has a half-life of ~0.7 ms.

Why filled shells resist chemistry

Each noble gas has a closed valence shell: He is 1s², Ne is [He]2s²2p⁶, Ar is [Ne]3s²3p⁶, Kr is [Ar]3d¹⁰4s²4p⁶, Xe is [Kr]4d¹⁰5s²5p⁶, Rn is [Xe]4f¹⁴5d¹⁰6s²6p⁶. Adding an electron would mean populating the next subshell where shielding from the closed core is high and effective nuclear charge low — so noble gases have small (or even positive) electron affinities. Removing an electron costs the largest first ionization energy in each row of the periodic table; He's IE₁ = 2372 kJ/mol is the largest of any element. Both routes to bonding are blocked, which is why Group 18 was thought inert until 1962.

Bartlett's insight was that "inert" depends on the strength of the partner. PtF₆, an extraordinarily powerful oxidizer, has an electron affinity high enough (~770 kJ/mol) to ionize Xe (IE₁ 1170 kJ/mol) when the lattice energy of the resulting [Xe⁺][PtF₆^-] salt makes up the deficit. Xenon's larger atomic size, higher polarizability, and lower IE compared to lighter Group 18 elements make it the most chemically tractable noble gas. Krypton compounds are limited (KrF₂ and a handful of others); argon, neon, and helium chemistry essentially requires cryogenic matrix isolation if it exists at all.

The fluorine atom is essential for these compounds because the X-F bond strengths are high (~150 kJ/mol per Xe-F), the F atom is small enough to fit five or six around a central Xe, and the strong electron-withdrawing nature of F stabilizes the positive xenon center. XeF₂ is linear (3-center 4-electron bonding), XeF₄ is square planar, and XeF₆ is a distorted octahedron with a stereochemically active lone pair. XeO₃ is pyramidal; XeO₄ is tetrahedral. All higher xenon oxides are explosive.

Noble-gas comparison table

Element	Boiling point (K)	Atomic radius (pm)	First ionization energy (kJ/mol)	Known compounds	Atmospheric abundance (ppm by volume)
He (Z = 2)	4.22	31	2372	HeH+ (transient); no neutral compounds at STP	5.24
Ne (Z = 10)	27.1	38	2081	None at STP; matrix species only	18.18
Ar (Z = 18)	87.3	71	1521	HArF in cryogenic matrix below 17 K	9340
Kr (Z = 36)	119.9	88	1351	KrF2 (decomposes above -45 °C), HKrF	1.14
Xe (Z = 54)	165.0	108	1170	XeF2, XeF4, XeF6, XeO3, XeO4, XePtF6, XeCl2	0.087
Rn (Z = 86)	211.5	120	1037	RnF2; chemistry limited by short half-life	~10-15 (trace)

Phases and uses

Element	Color of discharge	State at 25 °C	Key practical use	Production scale
He	Pale peach (yellow-orange)	Gas; superfluid below 2.17 K	Cryogenic NMR/MRI, balloons, leak testing	~30 kt/yr globally
Ne	Bright orange-red	Gas	Neon signs, vacuum tubes, helium-neon lasers	~500 t/yr
Ar	Pale violet	Gas	Inert atmosphere, lamp filling, MIG/TIG welding	~70 Mt/yr
Kr	Whitish-blue	Gas	Plasma display panels, KrF excimer lasers (248 nm)	~50 t/yr
Xe	Bluish-violet	Gas	HID headlights, anesthesia, ion thrusters (Hall effect)	~10 t/yr
Rn	Bright pink-red (alpha-induced glow)	Gas (radioactive)	Geological tracer; never used industrially	n/a (decay product only)

Applications and examples

• Bartlett 1962 XePtF₆. The first noble-gas compound, prepared at UBC. Bartlett mixed Xe with PtF₆ vapor and obtained an orange-yellow solid; he formulated it as Xe[PtF₆]. Modern crystallography revised this to XeF[PtF₅]·PtF₅, but the historical breakthrough stands. Bartlett's 1962 paper has been cited > 1000 times.
• Xenon anesthesia. Xe at ~70% partial pressure produces general anesthesia with stable hemodynamics; approved in Russia and Germany. Fast onset and recovery (blood-gas partition 0.115). Cost is the main constraint — atmospheric Xe is 0.087 ppm and recovery from spent gas is required for clinical economics.
• Krypton plasma display panels. AC plasma displays from 1995 to ~2010 used Ne-Xe-Kr mixtures excited by AC discharge, exciting xenon to emit UV that drove RGB phosphors. Largely displaced by LCD and OLED, but the Kr-Xe excimer chemistry remains relevant for excimer lamps used in semiconductor cleaning.
• Helium superfluidity. Below the lambda point at 2.17 K, He-4 has zero viscosity and Rollin film climbing. Discovered 1937-38 (Kapitsa, Allen, Misener). The two-fluid model (Landau, Nobel 1962) explains the absence of viscous drag. He-3 also superfluids but at 2.5 mK by Cooper-pair formation analogous to BCS superconductivity (Lee, Osheroff, Richardson, Nobel 1996).
• Ion-thruster propulsion. Xenon and increasingly krypton are propellants for Hall-effect thrusters and gridded ion engines on commercial geostationary satellites and deep-space probes (Dawn, Hayabusa, BepiColombo). Specific impulse 1500-3000 s — order of magnitude better than chemical propulsion. Starlink V2 satellites use krypton thrusters at ~1500 s I_sp.