What is so rare it has never been seen directly, because if you could get enough of it together, it would self-vaporize from its own radioactive heat?

“At” stands for astatine. It is an element with 85 protons packed into its nucleus, thus the atomic number “85” …

The problem is, there’s something about 85 protons in a tight space that nature doesn’t enjoy. Almost as soon as they squeeze together bits of nuclear material get spat out, or get added, and poof!It isn’t astatine any longer.

“This element has a half life of roughly 8 hours, meaning if you could get a clump of it to stay on a table (you can’t), half of it would disintegrate in 8 hours, and then every 8 hours another half would go until in a few days, there’d be no astatine on the table. Its nickname should be “Goodbye!”“

“Some of you will say that another element, francium (atomic number 87), is even more unstable than astatine, and you’re right. Its nickname should be “gone!” As Sam writes, “If you had a million atoms of the longest-lived type of astatine, half of them would disintegrate in 400 minutes. A similar sample of francium would hang on for 20 minutes. Francium is so fragile, it’s basically useless.” Astatine, however, is rarer and can be used to treat thyroid cancers.”

amazing science…

linkage

http://www.npr.org/blogs/krulwich/2013/07/12/201481293/the-hardest-thing-to-find-in-the-universe

What colour is it?

Most of the physical properties of astatine have been estimated (by interpolation or extrapolation), using various theoretically-grounded or empirically-derived methods. As an example, heavier halogens are darker than are halogens of lesser atomic weight – fluorine is nearly colorless, chlorine is bright green, bromine is brown, and iodine is dark gray/violet. Astatine is sometimes described as being a black solid (assuming that it follows this trend), or as having a metallic appearance (if it is a metalloid or a metal)

wiki.

Lots of unstable elements out beyond 85 -

The red ones have half-lives ranging between a minute and 1 day.

The purple ones have half-lives less than a minute.

http://en.wikipedia.org/wiki/Island_of_stability