How Rare Is Astatine?
Executive Summary
Astatine is widely regarded as the rarest naturally occurring element in Earth's crust. Best estimates place the total amount present at any moment between less than 1 gram and roughly 28 grams. Every isotope decays quickly - the longest lived is At-210 at about 8.1 hours - so astatine must be made on demand in accelerators for research and medicine.
What is astatine
Astatine is element 85, a halogen below iodine. It occurs in nature only as transient decay products of uranium, thorium, and trace neptunium series nuclides. Because all isotopes are short lived, no macroscopic natural samples persist.
How rare is astatine in nature
Consensus classification: Astatine is the rarest naturally occurring element in the crust. This status appears across reference works and teaching resources.
How much exists at any given time
Published estimates differ because they depend on decay chain models and sampling depth:
- Less than 1 gram worldwide - a frequently cited figure in academic and education materials.
- Up to about 28 grams worldwide - a more liberal upper bound that appears in several references.
- ~50 milligrams in the top 1 km of crust - an illustrative estimate focused on near-surface rock, highlighting how sparse it is where geologists actually sample.
All three lines of evidence agree on the conclusion: even at the high end, the instantaneous global inventory would barely fill a teaspoon if it were stable, and it is not.
Where it occurs
Astatine is generated in place inside uranium and thorium ores as members of the natural decay series. The naturally occurring isotopes include At-215, At-217, At-218, and At-219, with half lives measured in seconds.
Why astatine is so scarce
- Short half lives - the most stable isotope, At-210, lasts about 8.1 hours, and the medically useful At-211 lasts 7.2 hours. Any atoms produced in nature vanish quickly.
- No stable isotopes - continuous production by decay is balanced by rapid decay, keeping the steady state inventory tiny.
- Geochemical behavior - as a heavy halogen, astatine can form volatile species and anions, but its minute concentrations and self-heating from decay prevent accumulation. Bulk properties are inferred rather than observed.
How scientists make astatine
For experiments and for medicine, astatine is synthesized in cyclotrons by irradiating bismuth targets. The dominant route for At-211 is the 209Bi(α,2n)211At reaction, followed by rapid thermal release and wet chemical separation.
Process engineering continues to improve. Recent work automated target dissolution and extraction chromatography to accelerate At-211 delivery within its short half-life window.
Medical use: why At-211 matters
At-211 is a high-value alpha emitter for targeted alpha therapy. Each decay yields one alpha particle with a short tissue range, which can deliver lethal doses to cancer cells while limiting collateral damage. Research output and clinical interest have grown in the last decade.
Appearance and chemistry at a glance
| Property | What we know | Implication |
|---|---|---|
| Element family | Halogen, Z = 85 | Forms astatides and interhalogens, though experiments are at tracer levels. |
| Longest lived isotope | At-210, ~8.1 h | All bulk measurements are time constrained. |
| Ionization energy | ~9.3175 eV measured at CERN ISOLDE | Modern laser spectroscopy helped pin down a fundamental constant. |
| Bulk appearance | Inferred - likely a very dark solid if macroscopic amounts could exist | Self-heating from decay would disrupt any sizable sample. |
FAQs
Is astatine the rarest naturally occurring element on Earth
Yes. Reference sources call astatine the rarest naturally occurring element in Earth's crust.
How much astatine is on Earth right now
Model-based estimates span under 1 gram to roughly 28 grams present at any moment. The spread reflects different assumptions about crustal depth and production rates in decay chains.
Why do sources disagree on the total grams
Because you cannot mine and weigh astatine. Scientists estimate a steady state inventory by balancing production from decay chains against radioactive decay. Vary the geologic model and you vary the result.
What is the longest lived astatine isotope
Astatine-210 at about 8.1 hours. Astatine-211, the medical workhorse, is about 7.2 hours.
Can you see a lump of astatine
No. Any macroscopic sample would heat itself by radioactive decay faster than it could be handled. Bulk properties are inferred rather than observed.
How is At-211 shipped
It is produced to order and transported under radiopharmaceutical rules to nearby hospitals or labs because the 7.2 hour half life leaves little time margin.