Astatine (At)
Astatine is the 85th element, a halogen in the p-block. Its standard atomic weight is 210.
Astatine
210
What the table records
- Atomic number
- 85
- Standard atomic weight
- 210
- Group
- 17
- Period
- 6
- Block
- p-block
- Category
- Halogen
- [Xe] 4f14 5d10 6s2 6p5
- Valence electrons
- 7
- Typical valence
- 1
- Van der Waals radius
- 2 Å
- Covalent radius
- 1.5 Å
Molecules containing astatine
How to read these numbers
The standard atomic weight is an average over the isotopes as they occur naturally, which is why it is rarely a whole number. It is the figure a molar mass calculation uses. Work in mass spectrometry wants the monoisotopic mass of a single isotope instead, which is a different number.
The two radii are not the same measurement. The covalent radius is half the distance between two bonded atoms of the element; the van der Waals radius is how close a non-bonded atom gets before repulsion takes over, and is always the larger. Space-filling models are drawn with the second, ball-and-stick models with the first — which is why the same molecule looks so different in the two.