Astatine at a glance
- Atomic number
- 85
- Symbol
- At
- Atomic mass
- 210 u
- Category
- Halogen
- Group / period
- 17 / 6
- Electron configuration
- [Xe] 4f 14 5d 10 6s 2 6p 5
- State at 25 °C
- Solid
- Melting point
- 301.9 °C
- Boiling point
- 336.9 °C
- Density
- 7 g/cm³
- Electronegativity
- 2.2
- 1st ionization energy
- 899 kJ/mol
- Oxidation states
- −1, +1
- Discovered
- 1940
Atomic structure
A neutral astatine atom has 85 protons and 85 electrons, and its most common isotope carries about 125 neutrons. One atom weighs about 3.49 × 10⁻²⁵ kg, so a mole of astatine (6.022 × 10²³ atoms) weighs 210 g. That nucleus holds 1.47 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 7), which gives the configuration [Xe] 4f 14 5d 10 6s 2 6p 5. With 7 electrons in its outer shell, it tends to gain or share electrons to complete its outer shell. All of its isotopes are radioactive, so any astatine on Earth is either freshly made or part of a decay chain.
Physical properties
At room temperature (25 °C) astatine is a solid. It melts at 301.9 °C and boils at 336.9 °C, so it stays liquid across a 35-degree range. It stays solid in a hot kitchen oven but would melt in a candle flame (about 1000 °C). Its density is 7 g/cm³, 7 times as dense as water.
Chemical behaviour
Its electronegativity is 2.2 on the Pauling scale. Pulling off its outermost electron takes 899 kJ/mol, less than Iodine above it (1008 kJ/mol). A bond between astatine and chlorine would be polar covalent, with an electronegativity difference of 0.96. It releases 233 kJ/mol when it gains an extra electron. Its common oxidation states are −1, +1, so in compounds you will usually meet it as At⁻ and At⁺. Group 17 also holds Fluorine, Chlorine, Bromine, Iodine and Tennessine, which share its outer-shell pattern. Its Lewis symbol is At surrounded by 7 dots. Handle with care: as a pure element it is radioactive.
Where it comes from
It is vanishingly rare in nature, existing only briefly as part of radioactive decay chains. It was first identified in 1940, making it the 91st element to be discovered (counting the 11 known since antiquity together).