Atomic number 86 · Noble gas
Radon Rn
Radon (Rn), element 86, is a colourless, odourless, tasteless radioactive noble gas. It forms continuously from the decay of uranium and thorium in soil and rock and seeps into buildings; the World Health Organization ranks it second only to smoking as a cause of lung cancer.
Key properties
| Atomic number | 86 |
|---|---|
| Atomic weight | [222] (No stable isotope; mass number of a representative isotope in brackets) |
| Category | Noble gas |
| Group | 18 |
| Period | 6 |
| Block | p |
| State (25 °C, 1 atm) | Gas |
| Melting point | 202 K (-71.1 °C) |
| Boiling point | 211.45 K (-61.7 °C) |
| Density | 0.00973 g/cm³ |
| Electronegativity (Pauling) | — |
| Atomic radius (van der Waals) | 220 pm |
| First ionization energy | 10.745 eV |
| Electron affinity | — |
| Oxidation states | 0 |
| Discovered | 1900 |
Electron configuration
- Condensed
- [Xe]6s2 4f14 5d10 6p6
- Full
- 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 6s2 6p6
- Electrons per shell
- 2 · 8 · 18 · 32 · 18 · 8
Orbital diagram
Position in the table
Discovery
Radon was first known simply as an "emanation" in the early days of radioactivity research. In 1899 Ernest Rutherford and Robert Owens observed a radioactive gas given off by thorium compounds, and in 1900 Friedrich Ernst Dorn in Halle, Germany, reported a similar gas from radium. Dorn is usually credited with discovering radon (radon-222).
In 1910 William Ramsay and Robert Whytlaw-Gray isolated radon, measured its density and confirmed it was the heaviest known gas. They called it "niton", from the Latin for "shining", but "radon" became the accepted name around 1923.
Origin of the name
The name means the gas that comes from radium: radium plus the noble-gas ending -on, as in neon, argon and krypton.
Main uses
Radon has few practical uses.
- Historical medicine: in the early twentieth century radon sealed in small tubes was used to treat cancer. Some spas still advertise radon-rich water, but scientific evidence for health benefits is weak.
- Earth science: changes in radon levels in soil and groundwater are used to trace groundwater flow, to study possible earthquake precursors and to detect underground cavities.
- Atmospheric tracer: because radon comes mainly from land, its concentration shows whether an air mass has recently passed over continents.
Isotopes
All radon isotopes are radioactive. The most important, radon-222, forms when radium-226 decays in the uranium-238 chain and has a half-life of about 3.8 days. Radon-220 ("thoron") from the thorium series has a half-life of about 56 seconds, and radon-219 from the actinium series about 4 seconds. Most of the health risk comes not from radon itself but from its short-lived decay products, such as polonium, which lodge in the lungs and emit alpha particles.
In everyday life
Radon tends to build up in basements, ground floors and poorly ventilated rooms. Home test kits or local measurement services can check levels, and high readings can be reduced by ventilation, sealing floor cracks or installing sub-slab extraction. In South Korea in 2018, radon above the safety guideline was found in mattresses containing the mineral monazite, leading to a large recall and tighter controls on consumer products. Solid radon, below its melting point of about 202 K, is reported to glow through fluorescence.
Good to know
- Radon is the second leading cause of lung cancer after smoking.
- Apart from oganesson, radon is the heaviest noble gas.
- Most of the danger from radon comes from its decay products rather than the gas itself.
Same group (18)
Data sources · Properties: PubChem Periodic Table (US NIH/NLM public data) · Atomic weights: CIAAW 2024 abridged standard atomic weights · Names: Wikidata (CC0); Korean names follow the Korean Chemical Society. Retrieved 2026-09-23.