Radon
Atomic Data
| Atomic Number | 86 |
| Symbol | Rn |
| Atomic Weight | 222 u |
| Density (STP) | 9.73 g/L |
| Melting Point | −71.15 °C (202 K) |
| Boiling Point | −61.65 °C (211.5 K) |
| Electronegativity | : |
| Electron Config. | 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 6s2 6p6 |
| Oxidation States | 0, +2 |
| Phase at STP | Gas |
| Category | Noble Gas |
| Period / Group | 6 / 18 |
| CAS Number | 10043-92-2 |
Electron Configuration
[Xe] 4f14 5d10 6s2 6p6
| Shell | n | Subshell | Electrons | Cumulative |
|---|---|---|---|---|
| K | 1 | 1s | 2 | 2 |
| L | 2 | 2s | 2 | 4 |
| L | 2 | 2p | 6 | 10 |
| M | 3 | 3s | 2 | 12 |
| M | 3 | 3p | 6 | 18 |
| M | 3 | 3d | 10 | 28 |
| N | 4 | 4s | 2 | 30 |
| N | 4 | 4p | 6 | 36 |
| N | 4 | 4d | 10 | 46 |
| N | 4 | 4f | 14 | 60 |
| O | 5 | 5s | 2 | 62 |
| O | 5 | 5p | 6 | 68 |
| O | 5 | 5d | 10 | 78 |
| P | 6 | 6s | 2 | 80 |
| P | 6 | 6p | 6 | 86 |
| Total | 86 | 86 | ||
Isotopes of Radon
Radon is monoisotopic: ²²²Rn is its only naturally occurring stable isotope, accounting for 100% of all natural Radon.
| Isotope | Symbol | Protons | Neutrons | Abundance | Stability |
|---|---|---|---|---|---|
| Radon-222 | ²²²Rn | 86 | 136 | trace | Stable |
Abundance & Occurrence
Radon is present in Earth's crust at approximately trace amounts by mass and at approximately trace amounts by mass throughout the universe.
Earth's Crust (ppm by mass)
Universe (ppm by mass)
Discovery & History
Read more about the discovery of the periodic table of elements →
Safety & Handling
- Radioactive gas: lung cancer risk: Radon-222 (t½ = 3.8 days) decays to short-lived alpha-emitting progeny that deposit in the lungs; radon is the second leading cause of lung cancer after smoking, responsible for approximately 21,000 US deaths per year.
- Indoor accumulation: Radon seeps from soil and building materials and accumulates in basements, ground floors, and poorly ventilated spaces; the EPA recommends mitigation if indoor levels exceed 4 pCi/L (148 Bq/m3).
- Testing: Low-cost passive radon detectors can measure average annual radon levels; all homes: especially those in geologically susceptible areas: should be tested and, if necessary, fitted with sub-slab depressurisation systems.
- Occupational exposure: Underground miners (uranium, hard-rock), cave workers, and water treatment plant operators face elevated radon exposure; occupational radon monitoring and dose limits apply under national radiation protection regulations.
Radon in the Real World
Real-World Uses
- Geological and hydrological tracer: Radon-222 dissolved in groundwater and emanating from soil is used as a natural tracer for groundwater flow paths, aquifer recharge rates, and subsurface mixing processes in hydrogeology and environmental science.
- Earthquake prediction research: Anomalous radon emissions from soil and groundwater have been correlated with seismic activity in some studies; continuous radon monitoring is part of some earthquake precursor research programmes.
- Medical brachytherapy (historical): Radon gas sealed in gold needles was the first form of interstitial radiation therapy for cancer (early 20th century), exploiting its gamma-emitting daughter products; it has been completely replaced by safer sealed-source technologies.
- Radiation calibration: Radon-222 is used as a reference source for calibrating alpha-particle radiation detectors, radon-in-air measurement instruments, and environmental radioactivity monitoring equipment in national metrology institutes.
Downloadable Resources
Free periodic table reference sheets for classrooms, study sessions, and laboratory use.
Frequently Asked Questions
What is radon used for?
Radon's main uses are in medicine: radon-222 has been used in brachytherapy for cancer treatment, sealed into small gold seeds that are implanted in tumours. It is also used as a geophysical tracer; elevated radon levels in groundwater or soil gas can indicate seismic activity or geological faults. Some historical 'radon spas' claimed therapeutic benefits, but these are not medically recognised and radon is a known carcinogen.
Is radon dangerous?
Yes. Radon is the second leading cause of lung cancer after cigarette smoking, responsible for an estimated 21,000 deaths per year in the United States. Radon-222, a decay product of uranium, seeps from rocks and soil into buildings. In poorly ventilated spaces: particularly basements: it can accumulate to dangerous concentrations. Radon itself is a gas and quickly exhaled, but its short-lived radioactive decay products (polonium-218, lead-214, bismuth-214, polonium-214) can lodge in lung tissue and deliver alpha radiation.
How was radon discovered?
Radon was discovered in 1900 by German physicist Friedrich Ernst Dorn, who noticed that radium emitted a radioactive gas. William Ramsay and Robert Whytlaw-Gray isolated and characterised it in 1910 and named it niton (from the Latin 'nitens', glowing) for its faint phosphorescence. The name was changed to radon in 1923 after radium, the element it emanates from. It was the heaviest noble gas known until oganesson was synthesised.
How do you test for radon in a home?
Radon can be detected with inexpensive passive detectors: charcoal canisters are exposed for 2–7 days in the lowest liveable area of the home and mailed to a lab for analysis; long-term alpha track detectors are left for 3–12 months and give a time-averaged result. If levels exceed the action level (typically 148 Bq/m3 or 4 pCi/L in the US), mitigation involves installing a sub-slab depressurisation system: a pipe and fan that draws radon from below the foundation and vents it safely outside.