Radioactive Elements on the Periodic Table
Reviewed by Jerry Croteau, Founder & Editor
Table of Contents
Where Radioactivity Lives on the Table
Every element beyond bismuth (atomic number 83) is radioactive — none of its isotopes are permanently stable. But radioactivity isn't confined to the heavy end of the periodic table. Technetium (43) and promethium (61) also lack stable isotopes, despite sitting among otherwise stable neighbors. In total, approximately 38 elements have radioactive forms commonly encountered in nature, medicine, industry, or research.
The degree of radioactivity varies by many orders of magnitude. Potassium-40, present in every banana you eat, delivers a harmless dose of radiation. Polonium-210, just a few elements away from stable bismuth, is so intensely radioactive that a microgram on a fingertip can kill within weeks — it was the poison used to assassinate Russian dissident Alexander Litvinenko in London in 2006.
The Naturally Occurring Radioactive Elements
Uranium (U, 92) is the heaviest element with significant natural abundance. It's present in granite, phosphate rock, seawater, and ordinary soil at concentrations of 2-4 parts per million. You walk over uranium every day. Uranium-238, the dominant natural isotope at 99.3% abundance, has a half-life of 4.47 billion years — remarkably close to the age of Earth itself (4.54 billion years). This means roughly half of the uranium present when our planet formed from the solar nebula is still decaying today.
That decay generates heat. Uranium, thorium, and potassium-40 together produce an estimated 20 terawatts of geothermal power inside Earth — about half the planet's total internal heat budget. Without radioactive decay, Earth's interior would have cooled long ago, plate tectonics would have ceased, and the magnetic field that protects us from solar radiation would have collapsed.
Thorium (Th, 90) is three to four times more abundant than uranium in Earth's crust. It's found in monazite sands, common in India, Brazil, and Australia. Some nuclear engineers advocate thorium-based reactors as a safer, more proliferation-resistant alternative to uranium. Thorium fuel cycles produce far less long-lived waste and cannot easily be diverted to weapons production. India, which sits on one of the world's largest thorium reserves, has invested decades of research into thorium reactor technology.
Radium (Ra, 88) was the first radioactive element to achieve public fame — Marie Curie isolated it in 1898 and spent years concentrating it from tons of pitchblende ore. Radium's intense radioactivity (it glows blue in the dark from exciting nitrogen in the air) made it a sensation. It was added to toothpaste, chocolate, and watch dials before its dangers were understood. The "Radium Girls" — factory workers who painted luminous watch dials and licked their brushes to maintain a fine point — suffered devastating bone cancers and jaw necrosis, leading to landmark occupational safety litigation.
Plutonium and the Synthetic Actinides
Plutonium (Pu, 94) barely exists in nature — a few atoms form spontaneously in uranium ore through neutron capture, but no meaningful natural deposit has ever been found. Plutonium is manufactured in nuclear reactors when uranium-238 absorbs a neutron and undergoes two beta decays to become plutonium-239.
Plutonium-239 has a half-life of 24,100 years — long enough to remain dangerously radioactive for hundreds of millennia, short enough to be intensely active. A baseball-sized sphere of plutonium (about 6 kg) is enough to make a nuclear weapon. The first plutonium was produced in December 1940 by Glenn Seaborg's team at Berkeley using a 60-inch cyclotron. By August 1945, it powered the "Fat Man" bomb dropped on Nagasaki.
Today, plutonium's most constructive application is powering deep-space probes. Radioisotope thermoelectric generators (RTGs) convert plutonium's decay heat directly into electricity. The Voyager 1 and 2 spacecraft, launched in 1977, are still transmitting data from interstellar space on plutonium power nearly 50 years later. The Curiosity and Perseverance Mars rovers also run on plutonium RTGs — solar panels would be insufficient in Martian dust storms.
Americium (Am, 95) has the most mundane but arguably most important application of any transuranic element: it's in your smoke detector. A tiny amount of americium-241 (typically 0.29 micrograms) ionizes the air between two plates inside the detector. When smoke particles enter and disrupt the ion current, the alarm triggers. Americium saves thousands of lives annually and costs a fraction of a cent per unit.
The Superheavy Elements: 104 Through 118
Everything from rutherfordium (104) through oganesson (118) is entirely synthetic. These elements are created by accelerating lighter atoms to tremendous speeds in particle accelerators and smashing them into heavy target atoms. When the nuclei fuse, a superheavy atom forms — briefly.
The half-lives are staggeringly short. Oganesson-294 (element 118, the heaviest known element) lasts about 0.7 milliseconds before alpha-decaying into livermorium. Darmstadtium (element 110) persists for roughly 10 seconds. Only about five atoms of oganesson have ever been produced, all at the Joint Institute for Nuclear Research in Dubna, Russia, using a calcium-48 beam fired at a californium-249 target.
Because so few atoms are produced — and they decay so quickly — the physical properties of superheavy elements are almost entirely theoretical. Density values, melting points, and states of matter for elements 104-118 are predictions based on relativistic quantum mechanical calculations, not experimental measurements. Our
The Island of Stability
Nuclear physicists have long theorized an "island of stability" — a region of superheavy nuclei with specific combinations of protons and neutrons (called "magic numbers") that might have unusually long half-lives. Predictions suggest elements around atomic number 114 (flerovium) and 120-126 might have isotopes lasting minutes, hours, or even years rather than milliseconds.
Current evidence is tantalizing but inconclusive. Flerovium-298 (if it could be produced) is predicted by some models to have a half-life measured in days. Reaching the island would require creating neutron-rich superheavy isotopes — a technical challenge that existing particle accelerators are not quite capable of achieving. The race to element 119 and 120 continues at laboratories in Russia, Japan, and Germany.
Half-Life: The Mathematics of Decay
Every radioactive isotope decays at a precise, predictable rate characterized by its half-life — the time for exactly half the atoms in any sample to undergo radioactive transformation. Half-lives span 40 orders of magnitude: from tellurium-128 at 2.2 x 10^24 years (longer than the age of the universe by a factor of 160 billion) to hydrogen-7 at 2.3 x 10^-23 seconds.
The math is exponential: after one half-life, 50% remains. After two, 25%. After three, 12.5%. After ten half-lives, less than 0.1% of the original material persists. Our
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