He, Ne, Ar, Kr, Xe, Rn — column 18 of the periodic table. Through most of the 20th century, these six elements were considered chemically inert, unable to form any compounds. The term "inert gases" (before they were renamed "noble gases") reflected this view. In 1962, a single paper overturned the dogma: Neil Bartlett synthesized XePtF₆. Today, more than 200 xenon compounds are known, and even helium and neon still resist — but maybe not for much longer.
Why inert: the octet rule
Ground-state electron configurations of noble gases:
| Element | Configuration | 1st E_ionization (kJ/mol) | Pauling EN |
|---|---|---|---|
| He | 1s² | 2372 | — |
| Ne | [He] 2s² 2p⁶ | 2081 | — |
| Ar | [Ne] 3s² 3p⁶ | 1521 | — |
| Kr | [Ar] 3d¹⁰ 4s² 4p⁶ | 1351 | 3.00 |
| Xe | [Kr] 4d¹⁰ 5s² 5p⁶ | 1170 | 2.60 |
| Rn | [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p⁶ | 1037 | — |
All have a complete outer shell (s² p⁶, except He which has just 1s²). This configuration is extremely stable because:
1. All bonding orbitals are filled — no room to gain an electron without placing it in a higher shell (major energy cost). 2. All antibonding orbitals are empty — no easy site for a bonding partner. 3. The ionization energy is higher than all neighbors in the table (graph: Ne, Ar, Kr, Xe are all peaks).
Immediate consequence: noble gases don't form cations or anions easily, and don't engage in conventional covalent bonding. That's exactly the definition of chemical inertness.
The paradox: why not absolutely inert
But looking at the table above, you see that ionization energy decreases down the group: He 2372 → Xe 1170 → Rn 1037. Xenon ionizes almost as easily as oxygen (1314 kJ/mol). Yet it was considered inert because no one had found an oxidant strong enough to force the reaction.
Neil Bartlett, in 1962, noticed a crucial detail.
Bartlett 1962: the breakthrough
Bartlett, a young chemist at the University of British Columbia, was studying fluorine compounds. The previous year he had synthesized an unusual compound: PtF₆ + O₂ → O₂⁺[PtF₆]⁻. PtF₆ had ripped an electron off O₂ to form the dioxygenyl cation O₂⁺. PtF₆ was thus an extremely powerful oxidant.
Bartlett then compared two numbers:
- Ionization energy of O₂: 1175 kJ/mol
- Ionization energy of Xe: 1170 kJ/mol
Almost identical. If PtF₆ could oxidize O₂, it should oxidize Xe. Bartlett mixed Xe + PtF₆ in a glass tube — the two gases reacted immediately at room temperature, giving a yellow-orange solid: Xe⁺[PtF₆]⁻, the first xenon compound. The paper appeared in Proceedings of the Chemical Society in June 1962. Noble-gas chemistry was born.
The aftermath
Within six months, chemists worldwide produced:
- XeF₂ (Argonne National Laboratory, USA) — gentle fluorination of Xe at 400 °C.
- XeF₄ (Argonne) — strong fluorination with excess F₂.
- XeF₆ (Argonne) — extreme fluorination.
- XeO₃ (1963) — shock-explosive, no flame needed.
- XeOF₄, XeO₂F₂ — xenon oxofluorides.
- KrF₂ (1965) — first krypton compound, unstable above −20 °C.
Today (2025), more than 200 xenon compounds are known, including organic ones (CH₃-Xe-F, F-Xe-CN). Krypton has about twenty, radon is too radioactive for practical chemistry (KrF₂ and RnF₂ are the simplest observed).
What about helium, neon, argon?
Neon (Ne, IE = 2081 kJ/mol) still has no true chemical compound in 2025 — ionization is too costly, and its atom is too small to stabilize a charge.
Argon entered late in 2000: HArF was synthesized in Helsinki by Räsänen (solid matrix at 8 K). Compound unstable above 17 K, but it's the first observed Ar-X bond.
Helium (IE = 2372 kJ/mol — highest in the table) was long a holy grail. In 2017, an international team published in Nature the synthesis of Na₂He at 113 GPa — an ionic compound stabilized by extreme pressure effects. Not really a "chemical compound" in the classical sense (you need Jupiter-core conditions to form it), but proof of principle that even He can enter a crystal lattice with observable chemical behavior.
Why it matters
Beyond curiosity, xenon compounds have concrete applications:
- XeF₂: selective fluorination agent in organic synthesis (gentler than F₂).
- Excimer lasers XeCl, XeF, ArF, KrF: used in chip lithography (193 nm for ArF, 248 nm for KrF) — all modern processors are etched thanks to them.
- XeF₆: precursor for analytical chemistry of actinides.
- Medical imaging: hyperpolarized ¹²⁹Xe for lung MRI (airflow visualization).
What it changes
The Bartlett episode is one of the most instructive cases in philosophy of science. A universal dogma — "noble gases form no compounds" — fell within weeks because a chemist took two similar numbers (1175 and 1170) seriously and tried the experiment. The lesson is clear: chemical inertness isn't an absolute property, it's a measurement relative to the available oxidant. Raise the bar (PtF₆, O₂F⁺, KrF⁺), and even the "most stable elements in the table" become reactive.
Chemistry education in 2026 still teaches "noble gas = inert" as a first approximation — that's correct for 99 % of situations. But you have to know its origin to understand it's just an approximation.