Rn · Element 86 · Noble gas
A radioactive noble gas that seeps out of granite and is the second leading cause of lung cancer.
Structure
Not a diagram of dots on rings — a Monte-Carlo sample of the actual probability density |ψ|² for each occupied subshell. Drag to rotate. Blue and violet mark opposite signs of the wavefunction, which is what makes bonding possible.
Measured values
Every bar shows where radon sits among all 118 elements for that property.
Sources: IUPAC 2021 standard atomic weights · CRC Handbook of Chemistry and Physics · NIST. Values marked ~ are predicted rather than measured.
Same element, different neutrons
All radon atoms have the same number of protons, 86, and differ only in their number of neutrons: those variants are its isotopes. None of them is stable. The longest-lived known isotope, Rn-222, has 136 neutrons and a half-life of 3.8235 days.
| ISOTOPE | Neutrons | Natural Abundance | Half-life | Decay Mode |
|---|---|---|---|---|
| Rn-209 | 123 | — | 28.8 min | ε/β⁺ (83%), α (17%) |
| Rn-210 | 124 | — | 2.4 hr | α (96%), ε/β⁺ (4%) |
| Rn-211 | 125 | — | 14.6 hr | ε (72.6%), α (27.4%) |
| Rn-220 | 134 | — | 55.6 sec | α |
| Rn-222 longest-lived | 136 | — | 3.8235 days | α |
| Rn-224 | 138 | — | 107 min | β⁻ |
Here the classroom shortcut, rounded atomic mass (222) minus atomic number, gives the same answer: 136.
Decay modes
Data: IAEA Nuclear Data Section, LiveChart of Nuclides, ground-state evaluations, retrieved October 10, 2026. Abundance is the share of atoms in the natural element; a dash means the isotope has no standard natural abundance.
Size, to scale
Radius 150 pm — that is 0.15 nm, so about 3.33 million of them side by side would span a millimetre.
Thermal range
Radon is liquid over a 9 K window, from 202 K to 211 K.
The story
Radon is a radioactive noble gas that can accumulate in buildings.
Radon was discovered in 1900 by German physicist Friedrich Ernst Dorn at the University of Halle, while studying the radioactive decay of radium. Dorn observed that radium samples continuously produced a radioactive gas that he initially called "radium emanation." This discovery was part of the early radiation research that followed Henri Becquerels discovery of radioactivity in 1896 and Marie and Pierre Curies work with radium.
Following Dorns initial discovery, scientists William Ramsay and Robert Whytlaw-Gray isolated radon in 1910 and determined its atomic weight, confirming it as a noble gas. They measured its density and found it to be the heaviest known gas at the time. The element was initially called "niton" (from Latin "nitens," meaning shining) due to its luminescent properties, but was renamed "radon" in 1923.
In the early 1900s, radon was considered valuable for its supposed therapeutic properties. Entrepreneurs established "radon spas" and marketed radon-infused water as health tonics. The discovery that radon could make materials luminescent led to its use in self-luminous paint for watch dials and instrument panels, an application that continued until the health risks became apparent in the 1960s.
The understanding of radons danger evolved slowly. Lung cancer rates among uranium miners were noted as early as the 1500s, but the connection to radon wasnt established until the 1950s. Comprehensive studies of radon health effects began in the 1970s, leading to the recognition of radon as a major public health threat and the development of modern testing and mitigation programs in the 1980s.
Applications
Radon has been used in "radon therapy" since the early 1900s (radon baths began at Jáchymov in 1906, and spas such as Bad Gastein still offer it), in which patients were exposed to radon gas or radioactive water containing radon for treating arthritis, hypertension, and other ailments. These treatments, now known to be extremely dangerous, were based on the misguided belief that small amounts of radiation were beneficial. Today, radon-222 is still occasionally used in very controlled medical research to study lung cancer mechanisms and test radiation protection methods.
Radon serves as a valuable tracer in atmospheric and geological research. Scientists use radon concentrations to study air mass movements, track pollution dispersion, and understand groundwater flow patterns. Radon-222 acts as a natural "fingerprint" for identifying air masses and studying atmospheric mixing processes. Its decay products help researchers understand particle physics and test radiation detection equipment.
Radon measurement has become a crucial tool in public health and real estate. Professional radon testing services use specialized detectors to measure radon concentrations in homes, schools, and workplaces. This application has created an entire industry focused on radon detection, mitigation, and prevention, protecting millions of people from this invisible health hazard.
Controlled radon sources are used to train radiation safety professionals, health physicists, and environmental scientists. These applications help develop and test radon detection equipment, calibrate monitoring instruments, and research new mitigation techniques. Radon also serves as a model system for studying the behavior of other radioactive gases in the environment.
The most common modern use of radon technology involves professional testing services that help homeowners and businesses identify dangerous radon levels. Short-term tests (2-90 days) and long-term tests (over 90 days) use activated charcoal detectors, alpha track detectors, or electronic continuous monitors to measure radon concentrations. This industry employs thousands of certified professionals and generates hundreds of millions in annual revenue while protecting public health.
Understanding radon behavior drives the design of mitigation systems including sub-slab depressurization, crawl space ventilation, and air exchange systems. Engineers use radon source modeling to design effective systems that reduce indoor concentrations below EPA action levels (4 pCi/L). These systems protect millions of homes and are required by building codes in many high-radon areas.
Government agencies and research institutions use radon monitoring networks to track environmental trends, study climate change effects, and identify geological hazards. Radon has been studied as a possible earthquake precursor, but no method, radon included, has reliably predicted a major earthquake. These applications contribute to both public safety and scientific understanding of Earth processes.
Unlike historical uses, radon is no longer used in consumer products due to its severe health risks. Former applications included gold radon seeds implanted to treat cancer and radon-emitting health devices; luminous dial paint used radium, radon's parent, not radon. Today, any commercial use of radon is highly regulated and limited to specialized research applications in controlled environments.
Where it comes from
0 mg/kg of Earth's crust · more abundant than 0% of elements
Radon occurs naturally in virtually every building on Earth, originating from the radioactive decay of uranium-238 present in soil, rock, and building materials. Concentrations vary dramatically based on local geology, with particularly high levels in areas with granite bedrock, phosphate deposits, or uranium-rich soils. The EPA estimates that 1 in 15 U.S. homes has elevated radon levels, making it the second leading cause of lung cancer after smoking.
Radon-222 forms continuously in the uranium decay chain, starting with uranium-238 in rocks and soil. High-radon areas include the Reading Prong (Pennsylvania, New Jersey, New York), parts of Colorado, Iowa, and North Dakota. Radon can also originate from well water, particularly in areas with granite bedrock or uranium-rich aquifers. Some building materials, including concrete made with uranium-bearing aggregate, can be significant indoor radon sources.
Outdoor radon concentrations are typically low (0.1-0.4 pCi/L) due to atmospheric dilution, but indoor levels can be 10-100 times higher due to accumulation in enclosed spaces. Radon enters buildings through foundation cracks, sump pumps, floor drains, and gaps around pipes. Weather conditions, soil moisture, and barometric pressure changes significantly affect radon entry rates and indoor concentrations.
Radon dissolves readily in groundwater, particularly in areas with uranium-bearing bedrock. When radon-contaminated water is used for showering, washing, or drinking, the gas can be released into indoor air or ingested directly. The EPA estimates that radon in water causes about 168 deaths annually, with the highest risks in private wells drawing from granite or uranium-rich aquifers.
Handling
Radon is radioactive. It has no stable isotope — every nucleus decays. Handling requires appropriate shielding and licensing.
Radon is the second leading cause of lung cancer in the United States, responsible for approximately 21,000 deaths annually. When inhaled, radon decay products (polonium-218 and polonium-214) attach to lung tissue and emit alpha radiation, causing DNA damage that can lead to cancer. The risk increases dramatically for smokers, as tobacco smoke and radon exposure have synergistic effects on lung cancer development.
The EPA recommends testing all homes for radon, as levels can vary dramatically even between neighboring houses. The EPA recommends fixing a home when radon is 4 pCi/L (150 Bq/m3) or more and considering mitigation between 2 and 4 pCi/L, because no level is considered completely safe. Children are at higher risk due to their higher breathing rates and longer expected lifespans. Schools and workplaces should also be tested regularly.
Effective radon mitigation includes sub-slab depressurization systems, improved ventilation, and sealing foundation cracks. Professional installation is recommended, as improper mitigation can actually increase radon levels. Post-mitigation testing is essential to verify system effectiveness. Homeowners should also test water supplies, particularly private wells, as radon can be released during showering and washing.
Radon levels fluctuate seasonally and daily due to weather conditions, soil moisture, and barometric pressure changes. Long-term testing (over 90 days) provides the most accurate assessment of average exposure. Electronic continuous monitors can track daily variations and help identify factors that influence radon entry. Retesting at least every two years is recommended, and again after home renovations.
Quick answers
Radon (symbol Rn) is element 86 on the periodic table, a noble gas in period 6, group 18. A radioactive noble gas that seeps out of granite and is the second leading cause of lung cancer. At room temperature it is a gas, and it is radioactive.
[Xe] 4f¹⁴ 5d¹⁰ 6s² 6p⁶, giving 6 occupied shells with the distribution 2, 8, 18, 32, 18, 8. Its outer shell holds 8 electrons, which is what sets its bonding behaviour.Radon melts at 202 K (-71.1 °C) and boils at 211.3 K (-61.8 °C).
Radon has no stable isotope, so it has no standard atomic weight. The figure quoted, 222, is the mass number of its longest-lived known isotope.
Radon has no stable isotope. Its longest-lived known isotope, Rn-222, has 136 neutrons (mass number 222 minus atomic number 86) and a half-life of 3.8235 days. The classroom shortcut, rounded atomic mass (222) minus atomic number, gives the same 136.
Radon is a gas at room temperature; at 0 °C and 1 atm its density is about 9.7 g/L, so it is heavier than air.
Radon has a Pauling electronegativity of 2.2. The scale runs from 0.70 (francium, the least greedy for electrons) to 3.98 (fluorine, the most). Values in this middle band tend to form covalent rather than strongly ionic bonds.
Radon was discovered in 1900 by Friedrich Ernst Dorn. Origin of the name: from radium, its parent element.
Radon does not occur naturally on Earth in any meaningful quantity — it is made in a reactor or an accelerator.
Yes. Radon has no stable isotope — every one of its nuclei decays. Trace amounts occur naturally as decay products of heavier elements.