Radon at a glance
- Atomic number
- 86
- Symbol
- Rn
- Atomic mass
- 222 u
- Category
- Noble gas
- Group / period
- 18 / 6
- Electron configuration
- [Xe] 4f 14 5d 10 6s 2 6p 6
- State at 25 °C
- Gas
- Melting point
- -71.15 °C
- Boiling point
- -61.85 °C
- Density
- 9.73 g/L
- Electronegativity
- 2.2
- 1st ionization energy
- 1037 kJ/mol
- Oxidation states
- 0, +2
- Discovered
- 1900
Atomic structure
A neutral radon atom has 86 protons and 86 electrons, and its most common isotope carries about 136 neutrons. One atom weighs about 3.69 × 10⁻²⁵ kg, so a mole of radon (6.022 × 10²³ atoms) weighs 222 g. That nucleus holds 1.58 neutrons for every proton, typical of heavy nuclei, which need extra neutrons to hold together against the protons' repulsion. The electrons fill 6 shells (K 2, L 8, M 18, N 32, O 18, P 8), which gives the configuration [Xe] 4f 14 5d 10 6s 2 6p 6. Its outer shell is full, which is why radon hardly reacts with anything. All of its isotopes are radioactive, so any radon on Earth is either freshly made or part of a decay chain.
Physical properties
At room temperature (25 °C) radon is a gas. It melts at -71.15 °C and boils at -61.85 °C, so it stays liquid across a 9.3-degree range. To turn liquid it has to be cooled below -61.85 °C, colder than the freezing point of water. Only 10 elements have a lower melting point. As a gas it weighs 9.73 g per litre, 8.1 times heavier than air. Its atoms have a radius of about 120 pm. In the solid, its atoms pack into a face-centred cubic lattice.
Chemical behaviour
Its electronegativity is 2.2 on the Pauling scale. Pulling off its outermost electron takes 1037 kJ/mol, less than Xenon above it (1170 kJ/mol). A bond between radon and chlorine would be polar covalent, with an electronegativity difference of 0.96. Its common oxidation states are 0, +2, so in compounds you will usually meet it as Rn²⁺. Group 18 also holds Helium, Neon, Argon, Krypton, Xenon and Oganesson, which share its outer-shell pattern. Its Lewis symbol is Rn surrounded by 8 dots. Handle with care: as a pure element it is radioactive.
Where it comes from
It makes up 4e-13 mg/kg of Earth's crust, the 88th most abundant element there. It was first identified in 1900, making it the 83rd element to be discovered (counting the 11 known since antiquity together).