This week's element is francium, which has the symbol Fr and the atomic number 87.

Francium is a highly radioactive alkali metal that is vanishingly rare in the wild. It is the most unstable naturally occurring element, and one of the most unstable lab-synthesised elements. To the best of my knowledge, no one has ever seen francium with the naked eye, and if they had seen it, they certainly would not live to tell the tale. In fact, at any given time, it is estimated that less than 30 grams of francium are present throughout the Earth's crust, which makes the discovery of this element – before the nuclear age – so remarkable.

So how did we even know that francium exists if no one has ever seen it, even unto this very day? Mendeleyev predicted the existence of element 87 because his periodic table had an empty slot for it. Scientists also knew that element 87 must exist because heavier elements (thorium and uranium) had already been discovered. The talented chemist, Henry G. J. Moseley provided a scientific argument for the existence of element 87 based on his work with x-rays. The evidence in support of this prediction triggered the scientific equivalent of an Easter egg hunt among chemists and physicists around the world.

Francium is the last naturally occurring element to be discovered. After several erroneous claims, it was finally discovered in 1939 by the French physicist, Marguerite Perey, who began her scientific career as a student of two-time Nobel Laureate, Marie Curie. But this discovery was fraught with controversy since a team of physicists working in Romania had probably detected the element a few years earlier, lurking in a sample of the mineral pollucite. This team proposed the name moldavium for element 87 to honour the region where they made their discovery. But that original claim ignited controversy since it was argued at the time that element 87 does not exist in nature, although we now know that it does, albeit in trace amounts.

The second world war effectively silenced this argument until 1946, when Perey renewed her claim to the discovery of this element. This time, her claim was unopposed, probably due to the political atmosphere in the aftermath of the war. Since element 87 is the most electro-positive of all the elements, Perey proposed to name it name cation. But some scientists (including Perey's adviser, Irène Joliot-Curie) hated this name – um, something to do with cats? So after much shouting, Perey then proposed the name francium to honour her native France (this, even though another element – gallium – had already been named for France). Perey's claim was formally recognised three years later.

Francium is a naturally occurring radioactive decay product that pops up in thorium and uranium ores before it decays into either astatine, radium, or radon. Francium has 34 known isotopes, all of which are radioactive with very short half-lives: the most common isotope, Fr-223, has the longest half-life of just 22 minutes.

Although francium remains hidden and its isotopes' fleeting half-lives make its chemistry nearly impossible to study, scientists are able to predict what its chemical properties are like based on what we know about its smaller sister elements found in the same row of the periodic table; lithium, sodium, potassium, rubidium, and cæsium. For example, based upon these elements' increasingly violent reactions with water, scientists are certain that francium would react explosively with water.

In this video, our favourite chemistry professor tells us about francium and discusses how its reactivity in water is related to that of the other elements in the row above it:

[video link]

Read more about the discovery of francium.

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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:

Radon: Rn, atomic number 86

Astatine: As, atomic number 85

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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GrrlScientist is an evolutionary biologist, ornithologist and freelance science writer who writes about the interface between evolution, ethology and ecology, especially in birds. She seriously considered pursuing her doctorate in chemistry, and has shared her fondness for this topic by teaching a number of university chemistry and biochemistry courses. You can follow Grrlscientist's work on her other blog, Maniraptora, and also on facebook, Google +, LinkedIn, Pinterest and of course, twitter: @GrrlScientist