Radon (Rn)
Radon (Rn) is a noble gas with atomic number 86; its most stable known isotope has mass number 222. At room temperature it is a gas that melts at -71 °C and boils at -62 °C.
Radon key facts
Radon at a glance
Identity
| Symbol | Rn |
|---|---|
| Atomic number (Z) | 86 |
| Atomic weight | [222] mass number of the most stable isotope |
| Category | Noble gas |
| Group | 18 — noble gases |
| Period | 6 |
| Block | p-block |
| CAS number | 10043-92-2 |
Atomic properties
| Electron configuration | [Xe] 4f14 5d10 6s2 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 |
| Electronegativity (Pauling) | 2.2 |
| First ionisation energy | 10.748 eV (1,037.1 kJ/mol) 2nd: 18.99 eV · 3rd: 29.4 eV |
| Covalent radius | 142 pm |
| Van der Waals radius | 220 pm |
Physical properties
| State at 25 °C | Gas |
|---|---|
| Melting point | -71 °C · -95.8 °F · 202.15 K |
| Boiling point | -61.7 °C · -79.1 °F · 211.45 K |
| Density | 9.73 g/L (gas at 0 °C, 1 atm) |
| Specific heat capacity | 0.094 J/(g·K) |
| Thermal conductivity | 0.004 W/(m·K) |
| Crystal structure | Face-centred cubic |
Isotopes & abundance
| Natural occurrence | From radioactive decay |
|---|---|
| Radioactive | Yes |
| Most stable isotope | 222Rn — half-life 3.821 days |
| Known isotopes | 39 |
| Abundance in Earth's crust | 0 mg/kg |
| Abundance in seawater | 0 mg/L |
Atomic weights: CIAAW Abridged Standard Atomic Weights 2024. Other properties: standard reference compilations, checked October 2026; predicted values are marked.
Radon electron configuration
Radon has 86 electrons. Written in short form, its ground-state configuration is [Xe] 4f14 5d10 6s2 6p6 — the electrons of the noble gas xenon, plus 4f14 5d10 6s2 6p6.Written out in full: 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 6s2 6p6.
As a member of group 18 it has 8 valence electrons in its outer shell (shell 6), which largely decide its chemistry — a full outer shell, which is why it is so unreactive.
Discovery and name
1899–1900 — Ernest Rutherford and Robert B. Owens; Friedrich Ernst Dorn · Canada / Germany.
From radium, which it is produced from.
Known for
A radioactive gas that seeps from rocks and soil; it is the second leading cause of lung cancer after smoking, so homes are tested for it.
Isotopes of radon
Radon has no stable isotopes. Its longest-lived known isotope is:
| Isotope | Protons | Neutrons | Half-life |
|---|---|---|---|
| 222Rn | 86 | 136 | 3.821 days |
39 isotopes of radon are known in our data set (half-lives as published in nuclear data tables).
Radon compared with group 18
The other elements in group 18 (noble gases), from top to bottom of the table:
| Element | Z | Atomic weight | Melting point | Boiling point | Density | Electronegativity |
|---|---|---|---|---|---|---|
| Helium | 2 | 4.0026 | — | -269 °C | 0.1786 g/L | — |
| Neon | 10 | 20.180 | -249 °C | -246 °C | 0.9002 g/L | — |
| Argon | 18 | 39.95 | -189 °C | -186 °C | 1.784 g/L | — |
| Krypton | 36 | 83.798 | -157 °C | -153 °C | 3.749 g/L | 3 |
| Xenon | 54 | 131.29 | -112 °C | -108 °C | 5.894 g/L | 2.6 |
| Radon | 86 | [222] | -71 °C | -62 °C | 9.73 g/L | 2.2 |
| Oganesson | 118 | [294] | — | — | 7 g/cm³ (pred.) | — |