Mirror, Mirror on the Wall, Who's the Rarest of Them All?

I was watching Snow White and the Seven Dwarfs (1937) with my grand-kids the other day and, being a chemist, I wondered not who was the "fairest in the land," but what element was the rarest in the land. I know, I know... a pretty boring thought but when you have seen these Disney movies over and over your mind wanders.

Anyway I did some research and it turns out that it depends. It depends on how you define the elements and what you mean by the rarest. First I need to define what I consider as an element for this discussion. In my head I wanted to know the rarest of the naturally occurring elements but even that is a question all its own. It is generally considered that any element with an atomic number of 92 (Uranium) or less is a naturally occurring element. This means it is not synthetically created but can be found on Earth in a natural state, either as the free element or in a compound. But there are a couple of elements with atomic numbers less than 92 that are not found naturally - specifically Promethium(61) and Technetium(43). But for my analysis, I will consider all elements from Uranium on down.

So now that we have decided what constitutes a naturally occurring element, what makes it rare? Well that is a tougher proposition. I want to study the quantity on Earth at any given time since the elements we are evaluating are all radioactive. But this does not acknowledge their place in the Universe where several are relatively abundant. Just to keep things simple and consistent, I am only considering the amount on Earth.

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Which Element Would You Choose?
Astatine, Polonium, Francium,
Promethium or Technetium

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Lets look at what we are left with. These assumptions narrow down our search to the following five elements - Astatine(At - 85), Polonium(Po - 84), Francium(Fr - 87), Promethium(Pm - 61) and Technetium(Tc - 43). We will review some facts about these elements individually.

Astatine:
A radioactive element with the symbol "At" and an atomic number of 85. It occurs naturally on the Earth only as the result of decay of heavier elements. It is also made by bombarding bismuth(83) with energetic alpha particles. Thirty two isotopes are known with the six having mass numbers from 214 to 219 being the only ones found naturally. The isotope with the longest half-life is ²¹⁰Astatine, lasting just 8.1 hours.

Astatine is the heaviest known halogen, with melting and boiling points higher than those of the lighter halogens. ²¹¹Astatine is the only isotope used commercially, being employed as an alpha emitter in medicine. Because of its short half-life and small particle run, Astatine is considered preferable to ¹³¹Iodine in the diagnosis of disease. Similar to Iodine, Astatine is collected by the thyroid gland but, unlike Iodine, it does not destroy the neighboring parathyroid gland. Less than 28 grams (1 oz) exists naturally on Earth at any given time.

Polonium:
A highly radioactive element and a metalloid with the symbol "Po" and an atomic number of 84. It was first discovered by Marie and Pierre Curie in 1898 and occurs in Uranium ores as a by product of nuclear decay. Polonium is in the Oxygen family, along with Sulfur, Selenium and Tellurium.

Polonium has 33 known isotopes, all radioactive. The most stable is ²⁰⁸Polonium with a half-life of 2.9 years. ²¹⁰Polonium is highly toxic, with a toxicity of about 250,000 times that of Hydrogen Cyanide (HCN). The most common production of Polonium is by synthetically making it. ²¹⁰Polonium is a decay product of ²¹⁰Bismuth. The main use of Polonium is to eliminate static charges in manufacturing. An estimated 100 grams of Polonium is produced yearly (by natural means).

Francium:
The densest of the Alkali Metals and also radioactive. It has an atomic number of 87 and a symbol "Fr." Francium was discovered by Marguerite Perey in France in 1939. Found in Uranium ore, only about 1 atom of Francium is found for every 1 × 10¹⁸ Uranium atoms. It is calculated that there is at most 30 gm of Francium on Earth at any time. Francium’s most stable isotope, ²²³Francium, only has a half-life of 22 minutes. Because Francium is mainly created synthetically for research, it has no known use outside the laboratory.

Promethium:
Has atomic number 61 and symbol "Pm." It is most useful in nuclear reactors as an alpha emitter. It is one of the rarest of the Rare Earth Metals, and has only been detected naturally in the constellation Andromeda. It has never been found naturally on Earth, being generated by bombarding enriched Uranium. Its most stable isotope is ¹⁴⁵Promethium with a half-life of 17.7 years. The only isotope used in industry is ¹⁴⁷Promethium, which is used in radioionizators and in atomic batteries for guided missiles.

Technetium:
Atomic number 43 and symbol "Tc," has five isotopes, the two most common being ⁹⁸Technetium and ⁹⁹Technetium. ⁹⁸Technetium is one of the most stable radioactive elements, with a half-life of 4.2 million years. ⁹⁹Technetium is synthetically produced by the beta decay of ⁹⁹Molybdenum. Only very minute amounts are found on Earth. Technetium's short-lived isotope ^99mTechnetium ("m" indicates a metastable nuclear isomer), with a half-life of 6 hours, is used in nuclear medicine for a wide variety of diagnostic tests.

How do we pick the one that is the "Rarest in the Land?" All are found in only the smallest of quantities. All are radioactive with most decaying quickly into other more stable elements. I think my vote would go to Francium since at most only 30 grams exists at any moment and its longest living isotope has a half life of only 22 minutes. Astatine is a close second. Technetium is the most unusual with one isotope that has a half life of 4.2 million years and it is chemically surrounded by stable elements on the Periodic Table. These five elements are definitely some of the rarest and most unusual in God's Chemistry Set.

Now which element would you choose?

Proverbs 24:3-4 (NIV) - "By wisdom a house is built, and through understanding it is established; ⁴ through knowledge its rooms are filled with rare and beautiful treasures."