Radon (Rn)
Element 86 · Noble gas · atomic mass [222] u · group 18, period 6
Electron shells: 2, 8, 18, 32, 18, 8
Radon is a radioactive noble gas that seeps out of uranium-bearing rock and accumulates in basements. It is the second leading cause of lung cancer after smoking.
Radon in homes causes more lung-cancer deaths each year than any cause except smoking.
- Atomic number
- 86
- Atomic mass
- [222] u
- Electron configuration
- [Xe] 6s² 4f¹⁴ 5d¹⁰ 6p⁶
- Group · period · block
- 18 · 6 · p
- Oxidation states
- 0
- Electronegativity
- Not measured
- Melting point
- -71.15 °C
- Boiling point
- -61.7 °C
- Density
- 0.00973 g/cm³
- State at 20 °C
- Gas
- Appearance
- Colourless gas
- Discovered
- Friedrich Ernst Dorn, 1900
86 protons, and 86 electrons when neutral
Mass number of the longest-lived isotope — no standard atomic weight exists
Noble gases
What Radon is used for
- Historically, radiotherapy sources
- Geological and earthquake research tracer
Atomic structure of Radon
A neutral radon atom has 86 protons and 86 electrons. Its most common isotope carries about 136 neutrons, which is where the mass number 222 comes from. Change the neutron count and you have a different isotope of radon; change the proton count and you have a different element entirely.
Those electrons fill shells from the inside out: 2, 8, 18, 32, 18, 8. In orbital notation that is [Xe] 6s² 4f¹⁴ 5d¹⁰ 6p⁶, leaving 8 valence electrons in the outer shell — the ones that do the chemistry.
3D electron orbitals
See where Radon's electrons actually are — a cloud of 26,000 points sampled from the probability density |ψ|², not a cartoon of a lobe.
Where Radon sits in the periodic table
Group 18, with full outer shells and almost no drive to react. Their inertness is exactly what makes them useful for shielding, lighting and cooling.
Discovery and naming
Radon was discovered by Friedrich Ernst Dorn in 1900. Its name comes from Radium, from which it decays, which is also why its symbol (Rn) does not match its English name.
Every isotope of radon is radioactive, so it has no standard atomic weight. Periodic tables show the mass number of its longest-lived isotope in square brackets instead.
Keep exploring
Atomic weight from IUPAC/CIAAW; physical properties from PubChem (US National Library of Medicine). Values that have never been measured are shown as “not measured” rather than estimated.
Related Calculators
Frequently Asked Questions
What is the atomic mass of Radon?
Radon has no stable isotope, so it has no standard atomic weight. Periodic tables instead show 222 in square brackets — the mass number of its longest-lived isotope.
How many protons, neutrons and electrons does Radon have?
Radon has 86 protons, because the atomic number is the proton count and that is what makes an atom Radon at all. A neutral Radon atom has the same number of electrons, 86. The neutron count varies by isotope; the most common one has about 136, since 222 − 86 = 136.
What is the electron configuration of Radon?
The electron configuration of Radon is [Xe] 6s² 4f¹⁴ 5d¹⁰ 6p⁶. Its electrons are arranged 2, 8, 18, 32, 18, 8 across 6 shells.
Is Radon a metal?
No. Radon is classified as a noble gas, on the right-hand side of the table where elements gain or share electrons rather than losing them.
What group and period is Radon in?
Radon is in group 18 (the noble gases), period 6, in the p-block. The group is its column and the period is its row.
What is the melting point of Radon?
Radon melts at -71.15 °C and boils at -61.7 °C. That puts it in the gases at everyday temperatures.
What is Radon used for?
Historically, radiotherapy sources, Geological and earthquake research tracer. Radon is a radioactive noble gas that seeps out of uranium-bearing rock and accumulates in basements.
Who discovered Radon, and where does its name come from?
Radon (Rn) was discovered by Friedrich Ernst Dorn in 1900. The name comes from Radium, from which it decays.
Is Radon radioactive?
Yes. Every isotope of Radon is radioactive — it has no stable form at all, which is why the periodic table shows its mass in square brackets rather than as a standard atomic weight.