Many of them have been estimated from its position on the periodic table as a heavier analog of fluorine, chlorine, bromine, and iodine, the four stable halogens.

Chemically, several anionic species of astatine are known and most of its compounds resemble those of iodine, but it also sometimes displays metallic characteristics and shows some similarities to silver.

[13] For example, halogens get darker with increasing atomic weight – fluorine is nearly colorless, chlorine is yellow-green, bromine is red-brown, and iodine is dark gray/violet.

[20] As an analog of iodine it may have an orthorhombic crystalline structure composed of diatomic astatine molecules, and be a semiconductor (with a band gap of 0.7 eV).

[30][31][32] Despite this controversy, many properties of diatomic astatine have been predicted;[33] for example, its bond length would be 300±10 pm, dissociation energy <50 kJ/mol,[34] and heat of vaporization (∆Hvap) 54.39 kJ/mol.

[48][c] However, official IUPAC stoichiometric nomenclature is based on an idealized convention of determining the relative electronegativities of the elements by the mere virtue of their position within the periodic table.

[63] This cation exists as a coordination complex in which two dative covalent bonds separately link the astatine(I) centre with each of the pyridine rings via their nitrogen atoms.

[65][72] Astatine may form bonds to the other chalcogens; these include S7At+ and At(CSN)−2 with sulfur, a coordination selenourea compound with selenium, and an astatine–tellurium colloid with tellurium.

[73] Astatine is known to react with its lighter homologs iodine, bromine, and chlorine in the vapor state; these reactions produce diatomic interhalogen compounds with formulas AtI, AtBr, and AtCl.

[56] Oxidation of the element with dichromate (in nitric acid solution) showed that adding chloride turned the astatine into a molecule likely to be either AtCl or AtOCl.

[55][71] In 1869, when Dmitri Mendeleev published his periodic table, the space under iodine was empty; after Niels Bohr established the physical basis of the classification of chemical elements, it was suggested that the fifth halogen belonged there.

Before its officially recognized discovery, it was called "eka-iodine" (from Sanskrit eka 'one') to imply it was one space under iodine (in the same manner as eka-silicon, eka-boron, and others).

He chose the name "dor", presumably from the Romanian for "longing" [for peace], as World War II had started five years earlier.

In 1947, Hulubei's claim was effectively rejected by the Austrian chemist Friedrich Paneth, who would later chair the IUPAC committee responsible for recognition of new elements.

Even though Hulubei's samples did contain astatine-218, his means to detect it were too weak, by current standards, to enable correct identification; moreover, he could not perform chemical tests on the element.

Berta Karlik and Traude Bernert were unsuccessful in reproducing his experiments, and subsequently attributed Minder's results to contamination of his radon stream (radon-222 is the parent isotope of polonium-218).

[90][f] In 1942, Minder, in collaboration with the English scientist Alice Leigh-Smith, announced the discovery of another isotope of element 85, presumed to be the product of thorium A (polonium-216) beta decay.

The reason for this was that at the time, an element created synthetically in "invisible quantities" that had not yet been discovered in nature was not seen as a completely valid one; in addition, chemists were reluctant to recognize radioactive isotopes as legitimately as stable ones.

Any astatine present at the formation of the Earth has long since disappeared; the four naturally occurring isotopes (astatine-215, -217, -218 and -219)[110] are instead continuously produced as a result of the decay of radioactive thorium and uranium ores, and trace quantities of neptunium-237.

The landmass of North and South America combined, to a depth of 16 kilometers (10 miles), contains only about one trillion astatine-215 atoms at any given time (around 3.5 × 10−10 grams).

[116] Astatine was first produced by bombarding bismuth-209 with energetic alpha particles, and this is still the major route used to create the relatively long-lived isotopes astatine-209 through astatine-211.

Astatine is only produced in minuscule quantities, with modern techniques allowing production runs of up to 6.6 gigabecquerels[119] (about 86 nanograms or 2.47×1014 atoms).

Synthesis of greater quantities of astatine using this method is constrained by the limited availability of suitable cyclotrons and the prospect of melting the target.

With cryogenic technology, microgram quantities of astatine might be able to be generated via proton irradiation of thorium or uranium to yield radon-211, in turn decaying to astatine-211.

To produce the bismuth target, the metal is sputtered onto a gold, copper, or aluminium surface at 50 to 100 milligrams per square centimeter.

[129][m] Pre-1985 techniques more often addressed the elimination of co-produced toxic polonium; this requirement is now mitigated by capping the energy of the cyclotron irradiation beam.

[133][134] Wet methods involve "multiple radioactivity handling steps" and have not been considered well suited for isolating larger quantities of astatine.

Polonium X-rays emitted as a result of the electron capture branch, in the range of 77–92 keV, enable the tracking of astatine in animals and patients.

[125][140][o] Animal studies show that astatine, similarly to iodine—although to a lesser extent, perhaps because of its slightly more metallic nature[109]—is preferentially (and dangerously) concentrated in the thyroid gland.

[141] Early research suggested that injection of astatine into female rodents caused morphological changes in breast tissue;[142] this conclusion remained controversial for many years.