Chapter 18: The Group 18 Elements (Noble Gases)

Part A: The Essentials

18.1 The Elements

Key Point: Of the noble gases, only xenon forms a significant range of compounds with fluorine and oxygen.

The Group 18 elements—helium, neon, argon, krypton, xenon, and radon—are all monatomic gases. They are the least reactive elements and have been called the rare gases, inert gases, and currently noble gases. The name 'noble gases' is now accepted because it gives the sense of low but significant reactivity.

Their unreactivity can be understood from their ground-state valence electron configurations, ns²np⁶, their high ionization energies, and negative electron affinities.

He

Helium

Z = 2

Ne

Neon

Z = 10

Ar

Argon

Z = 18

Kr

Krypton

Z = 36

Xe

Xenon

Z = 54

Rn

Radon

Z = 86

Properties of the Noble Gases

Property	He	Ne	Ar	Kr	Xe	Rn
Atomic radius / pm	99	160	192	197	217	240
Melting point / °C	−272	−249	−189	−157	−112	−71
Boiling point / °C	−269	−246	−186	−152	−108	−62
Electron affinity / kJ mol⁻¹	—	−48.2	−115.8	−96.5	−96.5	−77.2
First ionization energy / kJ mol⁻¹	2373	2080	1520	1350	1170	1036

Atmospheric Abundance

0.72

He

1.26

Ne

3.97

Ar

0.06

Kr

−1.07

Xe

−6

Rn

Figure 18.1: Abundances of the noble gases (log atmospheric ppm by volume)

Helium makes up 23% by mass of the Universe but is rare in the atmosphere because its atoms travel fast enough to escape Earth's gravity. Argon (0.94% by volume) is abundant. Radon is radioactive and accounts for ~50% of background radiation.

18.2 Simple Compounds

Key Point: Xenon forms fluorides, oxides, and oxofluorides.

The most important oxidation numbers of Xe are +2, +4, and +6. Compounds with Xe−F, Xe−O, Xe−N, Xe−H, Xe−C, and Xe−metal bonds are known.

Xenon Fluorides

XeF₂

Linear

━━●━━

Oxidation: +2

XeF₄

Square Planar

⬜

Oxidation: +4

XeF₆

Distorted Octahedral

⬡

Oxidation: +6

Xenon Oxides & Oxofluorides

XeO₃

Trigonal Pyramidal

⚠ Explosive!

XeO₄

Tetrahedral

⚠ Explosive!

XeO₆⁴⁻

Octahedral

Perxenate ion

XeOF₂

T-shaped

XeO₃F₂

Trigonal Bipyramidal

XeOF₄

Square Pyramidal

Xenon forms hydrides in solid noble gases (HXeH, HXeOH, HXeOXeH) and clathrates with water (E·6H₂O).

Part B: The Detail

18.3 Occurrence and Recovery

Key Point: The noble gases are monatomic; radon is radioactive.

Etymology:

Helium - Greek helios 'sun'
Neon - Greek neos 'new'
Argon - Greek argos 'inactive'
Krypton - Greek kryptos 'hidden'
Xenon - Greek xenos 'strange'
Radon - named after radium

Helium-II (Superfluid):

When ⁴He is cooled below 2.178 K it becomes helium-II, a superfluid that flows without viscosity.

Box 18.1: Helium Demand

Liquid helium cools superconducting magnets for NMR/MRI. The Large Hadron Collider uses 96 tonnes. Demand is outstripping supply—some predict depletion in 20–30 years.

18.4 Uses

Key Point: Helium is used as an inert gas, in lasers, and as a cryogenic refrigerant.

🎈

Helium - Buoyancy

Low density and nonflammability for balloons.

🧲

Helium - Cryogenics

Coolant for superconducting magnets in NMR/MRI.

🤿

Helium - Diving

He:O₂ 4:1 mix prevents 'bends'.

⚙️

Argon - Welding

Inert atmosphere for air-sensitive compounds.

💡

Light Sources

Neon signs, xenon flash lamps, lasers.

☢️

Radon - Hazard

Radioactive; ~50% of background radiation.

Box 18.2: ¹²⁹Xe-NMR Imaging

Hyperpolarized ¹²⁹Xe (enhanced 10⁵× via spin exchange) enables MRI of lungs, brain, and other organs.

18.5 Synthesis and Structure of Xenon Fluorides

Key Point: Xenon reacts with fluorine to form XeF₂, XeF₄, and XeF₆.

March 1962

Neil Bartlett observed the first noble gas reaction with PtF₆.

Shortly After

Rudolf Hoppe's group confirmed the findings, sparking worldwide research.

Within a Year

Xenon fluorides and oxo compounds were synthesized and characterized.

Xenon Fluoride Synthesis

Xe(g) + F₂(g) → XeF₂(g)

400°C, 1 atm, Xe excess

Xe(g) + 2 F₂(g) → XeF₄(g)

600°C, 6 atm

Xe(g) + 3 F₂(g) → XeF₆(g)

300°C, 60 atm

"Windowsill" Synthesis

Xe + F₂ sealed in dried glass and exposed to sunlight slowly forms XeF₂ crystals via photodissociation of F₂.

18.6 Reactions of Xenon Fluorides

Key Point: Xenon fluorides are strong oxidizing agents; form complexes XeF₅⁻, XeF₇⁻, XeF₈²⁻.

XeF₆(s) + 3 H₂O(l) → XeO₃(aq) + 6 HF(g)

2 XeF₂(s) + 2 H₂O(l) → 2 Xe(g) + 4 HF(g) + O₂(g)

XeF₂(s) + SbF₅(l) → [XeF]⁺[SbF₆]⁻(s)

XeF₅⁻ is pentagonal planar. XeF₈²⁻ is a square antiprism (unusual for VSEPR—no site for lone pair).

18.7 Xenon–Oxygen Compounds

Key Point: The xenon oxides are unstable and highly explosive.

Xenon oxides are endergonic (Δ_fG° > 0); cannot form directly from elements.

Serious Hazard

XeO₃ is highly explosive with E°(XeO₃, Xe) = +2.10 V.

2 HXeO₄⁻ + 2 OH⁻ → XeO₆⁴⁻ + O₂ + 2 H₂O

Perxenates contain octahedral XeO₆⁴⁻. XeO₄ is explosively unstable.

Write a balanced equation for decomposition of xenate ions to perxenate ions, xenon, and oxygen.

18.8 Xenon Insertion Compounds

Key Point: Xenon can insert into H−Y bonds.

Noble-gas hydrides HEY are isolated by matrix isolation: HXeCl, HXeBr, HXeI, HKrCl, HXeOH, HXeOXeH, HXeCCH.

"Missing Xenon" Phenomenon:

Atmospheric Xe is depleted 20× vs. other noble gases. Theory: Xe forms stable compounds in Earth's interior under extreme conditions.

Box 18.3: Matrix Isolation

Reactive species are trapped in solid noble gas at low temperature under vacuum for spectroscopic study.

18.9 Organoxenon Compounds

Key Point: Organoxenon compounds are prepared by xenodeborylation of organoboron compounds.

First Xe−C compound: 1989. Main routes via XeF₂ and XeF₄.

(C₆F₅)₃B + 3 XeF₂ → [C₆F₅Xe]⁺ + products

RBF₂ + XeF₂ → [RXe]⁺ + [BF₄]⁻

Xe(II) compounds decompose above −40°C. Extended π systems and electron-withdrawing groups (F) stabilize Xe−C bonds.

18.10 Coordination Compounds

Key Point: Ar, Kr, Xe form coordination compounds; stability: Xe > Kr > Ar.

First stable noble-gas coordination compound: [AuXe₄]²⁺ (square planar). [Xe₂]⁺ has Xe−Xe = 309 pm. Linear Xe₄⁺ has the longest homonuclear main-group bonds (353, 319 pm).

Matrix isolation yields [Fe(CO)₄Xe] and [M(CO)₅E] (M = Cr, Mo, W; E = Ar, Kr, Xe).

18.11 Other Compounds of Noble Gases

Key Point: Krypton and radon fluorides exist but are less extensive than xenon chemistry.

KrF₂: linear, highly endergonic, prepared at −196°C. HArF: stable to 27 K. Clathrates form with Ar, Kr, Xe but not He or Ne (too small). Endohedral fullerenes: He@C₆₀ⁿ⁺.

Exercises

Explain why helium is rare in the atmosphere despite being the 2nd most abundant element in the universe.

Choose a noble gas for: (a) lowest-temperature refrigerant, (b) lowest ionization energy light source, (c) cheapest inert atmosphere.

Describe synthesis of (a) XeF₂, (b) XeF₆, (c) XeO₃.

Draw Lewis structures: (a) XeOF₄, (b) XeO₂F₂, (c) XeO₆²⁻.

Give noble-gas species isostructural with: (a) ICl₄⁻, (b) IBr₂⁻, (c) BrO₃⁻, (d) ClF.

(a) Lewis structure for XeF₇⁻. (b) Predict structure using VSEPR.

Calculate bond order of E₂⁺ (E = He, Ne) using MO theory.

Predict VSEPR structures: (a) XeF₃⁺, (b) XeF₃⁻, (c) XeF₅⁺, (d) XeF₅⁻.

Identify compounds A–E in the reaction scheme starting from Xe + F₂.

Predict the ¹²⁹Xe-NMR spectrum of XeOF₃⁺.

Predict the ¹⁹F-NMR spectrum of XeOF₄.

Tutorial Problems

18.1 Predicted Chemical Bonds Between Rare Gases and Au

Compare Au−Rg and H−Rg bond energies/lengths from Pyykkö's computational study (J. Am. Chem. Soc., 1995, 117, 2067).

18.2 Atypical Compounds of Noble Gases

Sketch XeF₅Cl, HXeOOXeH, ClXeFXeCl⁺. Explain XeF₂ as a ligand (Chem. Soc. Rev., 2007, 36, 1632).

18.3–18.8 Additional Tutorial Problems

Topics include: first Xe−N compound, matrix isolation techniques, [AuXe₄]²⁺ synthesis, XeOF₅⁻ characterization, He−C bond calculations, and superfluidity in solid helium.