This week, we meet the element astatine, whose very name tells us almost half of all that we know about it. Astatine's chemical symbol is At and its chemical number is 85. Its name comes from the Greek and means "unstable".

Ain't that the truth.

Since it was first discovered in 1940, astatine was thought to be the rarest of all naturally-occurring elements on Earth. However, this distinction was lost after the hair-splitting realisation that highly-concentrated uranium deposits are capable of producing a few measly atoms of berkelium.

As you can see in the above image ... hang on, you do see it, don't you? No? Well, that's because no one has ever seen astatine. In fact, the total amount of astatine present on Earth in any one year could easily fit on one of your laptop's keycaps. But if you had that amount of astatine sitting on your laptop at this moment, you would not be able to describe its appearance for us since it would immediately vaporize you, your laptop and office, along with a significant portion of the landscape surrounding you.

This means we are free to imagine astatine's appearance. Inspired by that most-huggable element, bismuth (and also by the lovely vanadium), I chose the above featured image as my idea of what astatine might look like. Your ideas may differ.

But being scientists, we can also do some chemistry sleuthing to get a more accurate mental image of astatine's appearance before it explodes and triggers a nuclear meltdown. Since this element is found below iodine in the periodic table, it is a halogen and thus, it shares many qualities with its smaller sister elements: like the other halogens, it's probably highly toxic, highly chemically reactive, and very electronegative -- aggressively stealing electrons from other elements.

But unlike its smaller sister elements, astatine is also highly radioactive, which is one of the reasons it does not appear in the above image. Since all isotopes of astatine have fleeting half-lives (astatine-210 has longest half-life, which is only 8.3 hours, whereas astatine-213's half-life is less than half a microsecond), and since this element only pops up momentarily in the aftermath of nuclear disintegration, it's unlikely that anyone ever will see it unless North Korea makes good on its penis willy-waving threats to "nuke America".

In this short video, our favourite chemistry professor fills us in on most of the remainder of humanity's collective knowledge about astatine:

[video link]

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Video journalist Brady Haran is the man with the camera and the University of Nottingham is the place with the chemists. You can follow Brady on twitter @periodicvideos and the University of Nottingham on twitter @UniofNottingham

You've already met these elements:

Polonium: Po, atomic number 84

Bismuth: Bi, atomic number 83

Lead: Pb, atomic number 82

Thallium: Tl, atomic number 81

Mercury: Hg, atomic number 80

Gold: Au, atomic number 79

Platinum: Pt, atomic number 78

Iridium: Ir, atomic number 77

Osmium: Os, atomic number 76

Rhenium: Re, atomic number 75

Tungsten: W, atomic number 74

Tantalum: Ta, atomic number 73

Hafnium: Hf, atomic number 72

Lutetium: Lu, atomic number 71

Ytterbium: Yb, atomic number 70

Thulium: Tm, atomic number 69

Erbium: Er, atomic number 68

Holmium: Ho, atomic number 67

Dysprosium: Dy, atomic number 66

Terbium: Tb, atomic number 65

Gadolinium: Gd, atomic number 64

Europium: Eu, atomic number 63

Samarium: Sm, atomic number 62

Promethium: Pm, atomic number 61

Neodymium: Nd, atomic number 60

Praseodymium: Pr, atomic number 59

Cerium: Ce, atomic number 58

Lanthanum: La, atomic number 57

Barium: Ba, atomic number 56

Cæsium: Cs, atomic number 55

Xenon: Xe, atomic number 54

Iodine: I, atomic number 53

Tellurium: Te, atomic number 52

Antimony: Sb, atomic number 51

Tin: Sn, atomic number 50

Indium: In, atomic number 49

Cadmium: Cd, atomic number 48

Silver: Ag, atomic number 47

Palladium: Pd, atomic number 46

Rhodium: Rh, atomic number 45

Ruthenium: Ru, atomic number 44

Technetium: Tc, atomic number 43

Molybdenum: Mo, atomic number 42

Niobium: Ni, atomic number 41

Zirconium: Zr, atomic number 40

Yttrium: Y, atomic number 39

Strontium: Sr, atomic number 38

Rubidium: Rr, atomic number 37

Krypton: Kr, atomic number 36

Bromine: Br, atomic number 35

Selenium: Se, atomic number 34

Arsenic: As, atomic number 33

Germanium: Ge, atomic number 32

Gallium: Ga, atomic number 31

Zinc: Zn, atomic number 30

Copper: Cu, atomic number 29

Nickel: Ni, atomic number 28

Cobalt: Co, atomic number 27

Iron: Fe, atomic number 26

Manganese: Mn, atomic number 25

Chromium: Cr, atomic number 24

Vanadium: V, atomic number 23

Titanium: Ti, atomic number 22

Scandium: Sc, atomic number 21

Calcium: Ca, atomic number 20

Potassium: K, atomic number 19

Argon: Ar, atomic number 18

Chlorine: Cl, atomic number 17

Sulfur: S, atomic number 16

Phosphorus: P, atomic number 15

Silicon: Si, atomic number 14

Aluminium: Al, atomic number 13

Magnesium: Mg, atomic number 12

Sodium: Na, atomic number 11

Neon: Ne, atomic number 10

Fluorine: F, atomic number 9

Oxygen: O, atomic number 8

Nitrogen: N, atomic number 7

Carbon: C, atomic number 6

Boron: B, atomic number 5

Beryllium: Be, atomic number 4

Lithium: Li, atomic number 3

Helium: He, atomic number 2

Hydrogen: H, atomic number 1

Here's the Royal Society of Chemistry's interactive Periodic Table of the Elements that is just really really fun to play with!

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