Periodic table of the chemical elements
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85 At,Astatine
Halogens · Solid (predicted)
Atomic mass [210] mass number of the most stable isotope
85
Average mass of an atom in unified atomic mass units (1 u is 1/12 of the mass of a carbon-12 atom). Accounts for the natural mix of the element’s isotopes.
Number of the vertical column, 1 to 18 in IUPAC numbering. Elements in a group share the same outer-shell layout, so their properties are alike.
Number of the horizontal row. Usually equal to the number of electron shells in the atom; palladium is the exception, with four shells in period 5.
The subshell (s, p, d or f) that receives the last electrons in the usual filling order; it divides the table into four blocks. Real atoms sometimes differ: lanthanum starts the f-block but has no f electrons.
How electrons are arranged in subshells. The bracket stands for the previous noble gas; the superscript is the number of electrons in a subshell.
Number of electrons in each shell, from the nucleus outwards.
Temperature at which the solid turns liquid at standard pressure. Helium, carbon and arsenic do not melt at normal pressure: helium needs about 25 atm, carbon and arsenic turn straight into vapour.
Temperature at which the liquid boils at standard pressure. For carbon and arsenic, the temperature at which the solid turns straight into vapour (sublimation).
Mass of one cubic centimetre of the substance. For gases, at 0 °C and standard pressure.
How strongly an atom pulls electrons towards itself in a chemical bond. On the Pauling scale it runs from 0.79 for caesium to 3.98 for fluorine.
Energy needed to remove the most loosely bound electron from an atom. The lower it is, the more easily the element gives up an electron, which is typical of metals.
Energy released when a neutral atom gains an electron. Highest for the halogens. Noble gases, nitrogen and some metals form no stable negative ion.
Half the length of a single covalent bond between two atoms of the element. It shows the size of an atom inside a molecule or crystal: 76 pm for carbon, 132 pm for iron.
Charges the atom would have in its compounds if all its bonds were ionic; the main ones are listed.
Astatine (At) is a chemical element with atomic number 85. It sits in group 17 and period 6 of the periodic table, in the p-block. It is a halogen. At room temperature it is predicted to be a solid. It has no standard atomic weight; the mass number of its most stable isotope is 210.
The name comes from the Greek astatos, ‘unstable’: all its forms decay within hours. For decades the gap below iodine was called ‘eka-iodine’, and several claimed discoveries in nature turned out to be wrong. In 1940 Dale Corson, Kenneth MacKenzie and Emilio Segrè at the University of California made the element by bombarding bismuth with alpha particles.
Astatine is the rarest naturally occurring element. It forms only briefly from the decay of uranium and thorium, and at any moment the whole Earth’s crust holds less than about 30 grams of it.
So little astatine can be made, and it decays so fast, that it is used only in research. Astatine-211 is being studied as a way to destroy cancer cells from inside the body.