What Is the Rarest Element on Earth?

The rarest naturally occurring element in the Earth’s crust is astatine, element 85. Estimates of how much exists at any moment vary between sources, but they agree that it’s tiny — often quoted as well under a gram in the entire crust, possibly only a few tens of milligrams.

Why astatine is so rare

Astatine has no stable isotopes. Its longest-lived isotope, astatine-210, has a half-life of only about 8 hours. So any astatine present when the Earth formed vanished billions of years ago.

The tiny amount that exists today is constantly being made and destroyed. When uranium and thorium in rocks decay, they pass through a long chain of radioactive intermediates, and a very small fraction of those chains briefly produce astatine atoms. Each one decays again within seconds to hours. The total at any moment reflects that balance between production and decay — which is why it’s so minuscule.

As a result, nobody has ever seen a visible sample of astatine. Everything we know about it comes from experiments on a few billion atoms at a time, made artificially in particle accelerators. Its name comes from the Greek astatos, “unstable”.

Researchers are interested in astatine-211 for targeted alpha therapy: attached to a molecule that seeks out cancer cells, it delivers a lethal dose of alpha radiation over a very short distance.

The other contender: francium

Francium, element 87, is also extraordinarily rare, for similar reasons: its longest-lived isotope, francium-223, has a half-life of only about 22 minutes. It forms occasionally in the decay of uranium-235 (via actinium-227). Estimates of the amount in the crust at any time are usually tens of grams. Like astatine, a weighable sample has never been assembled — the largest collection ever made, a few hundred thousand atoms held in a laser trap, was still invisible to the naked eye.

The rarest stable elements

If you only count elements with stable isotopes, the rarest in the Earth’s crust are metals in the platinum group, along with tellurium and gold. Rhenium, rhodium, iridium, osmium, ruthenium and tellurium are each present at roughly one part per billion or less of the crust. Gold is not far behind — all the gold ever mined would fit in a cube only about 22 metres on each side.

Rarity in the crust partly reflects how Earth formed. When the planet was young and molten, iron sank to form the core, and elements that dissolve readily in molten iron — the “siderophile” (iron-loving) elements like gold, platinum and iridium — were dragged down with it. Much of the gold and platinum near the surface today may have been delivered later by meteorite impacts.

Synthetic elements are rarer still

Elements beyond uranium are essentially absent from nature (apart from tiny traces of neptunium and plutonium). The superheavy elements, like oganesson and tennessine, have each been made only a few atoms at a time in laboratories, and those atoms decay in fractions of a second. At any given moment the Earth probably contains no atoms of them at all. See how new elements are made.

“Rare earth elements” aren’t rare

The rare earth elements — the lanthanides plus scandium and yttrium — are named for historical reasons, not scarcity. Cerium is about as abundant in the crust as copper, and even the rarest stable ones (thulium and lutetium) are more common than silver or gold. The exception is promethium, which has no stable isotopes and barely exists in nature. What makes them “rare” is that they’re spread thinly and chemically very similar, so they’re hard to find in concentrated deposits and hard to separate from each other. See rare earth elements.

The most common elements, for contrast

The Earth’s crust is dominated by oxygen (about 46% by mass) and silicon (about 28%), followed by aluminium, iron, calcium, sodium, magnesium and potassium. Those eight elements make up more than 98% of the crust. See the most abundant elements in the Earth’s crust.

Quick answers

What is the rarest element in the universe? Among natural elements, the short-lived ones like astatine and francium. Overall abundances fall steeply beyond iron; among stable elements, the heaviest (like the platinum-group metals) are among the rarest.

What is the rarest element you can buy? Among elements available for purchase in useful amounts, rhodium, osmium and iridium are among the scarcest. Californium, made in reactors, is sold in micrograms at extraordinary prices.

Is astatine dangerous? Yes — it’s intensely radioactive. But there’s so little of it that it poses no practical hazard outside a laboratory.