Is astatine a non-metal or a metalloid?

Question

Is astatine a non-metal or a metalloid?

A metalloid is an element which exhibits some of the properties of a metal as well as those of a non metal.

Answer 1

Is Astatine a non metal or a metalloid?

Unknown

According to the Wikipedia article on astatine, the longest-lived isotope has a half-life of 8.1 hours. The element is so radioactive that large samples would melt and vaporize under the heat generated by its own radioactivity. Such behavior makes it difficult to assess whether astatine has the appropriate traits to be a metalloid.

Metalloids tend to be lustrous, brittle, semiconductors with other properties intermediate between metals and nonmetals. Nonmetals are dull/matte, brittle, and poor conductors of electricity and heat. All of these properties require a visible solid (or liquid) sample to assess, and thus astatine is impossible to assess and classify.

Astatine is usually placed with the metalloids or the nonmetals by different authors based on its position in periodic table, but in truth, astatine is so unstable in its elemental form, we have no idea which one it is.

Answer 2

A metalloid, possibly a metal

Even though nobody has so far seen or been able to test a visible piece of astatine we can extrapolate its properties based on its position in the periodic table. We know that iodine, the element above astatine, has a metallic appearance when viewed under white light, is a semiconductor (in the direction of its planes), and that there is evidence of some delocalised bonding between iodine molecules in crystalline iodine.

Given its position in the periodic table astatine could reasonably be expected to show more metallic character than iodine. Consistent with this expectation, the most common oxidation state of astatine appears to be +1 whereas that of the lighter halogens is –1; and there is some evidence to suggest that astatine forms a cation in strongly acidic solutions. In 2013, based on relativistic modelling calculations, condensed astatine was predicted to be a monatomic metal with a face-centred cubic structure. Metals with this structure are noted for their ductility whereas metalloids are regarded as being brittle.

If astatine does turn out to be a metal then, like other post-transition metals, it could be expected to show appreciable non-metallic character. Trace level experiments on the chemistry of astatine show that it appears to act like a metal in some respects and like a non-metal in others.

The Wikipedia article on metalloids has more information about astatine and its metallic and non-metallic properties. For now, until more evidence comes in, it would be reasonable to classify astatine as a metalloid.