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Every element has at least some radioactive isotopes. But when people ask which elements are “radioactive”, they usually mean elements with no stable isotopes at all — where every atom will eventually decay. There are 38 of those among the 118 known elements.
The list
Two light elements with no stable isotopes:
- Technetium (element 43) — the lightest radioactive element
- Promethium (element 61)
Every element from polonium (84) onwards: 84 polonium, 85 astatine, 86 radon, 87 francium, 88 radium, 89 actinium, 90 thorium, 91 protactinium, 92 uranium, 93 neptunium, 94 plutonium, 95 americium, 96 curium, 97 berkelium, 98 californium, 99 einsteinium, 100 fermium, 101 mendelevium, 102 nobelium, 103 lawrencium, 104 rutherfordium, 105 dubnium, 106 seaborgium, 107 bohrium, 108 hassium, 109 meitnerium, 110 darmstadtium, 111 roentgenium, 112 copernicium, 113 nihonium, 114 flerovium, 115 moscovium, 116 livermorium, 117 tennessine, 118 oganesson.
That’s 2 + 35 = 37 — plus bismuth (83) in a technical sense, which makes 38. See below.
The bismuth surprise
For a long time bismuth was thought to have a stable isotope, bismuth-209. In 2003, French researchers detected that it does decay — by alpha emission, with a half-life of about 2 × 10¹⁹ years, more than a billion times the age of the universe. For every practical purpose bismuth is stable (it’s used in stomach medicines and cosmetics), but strictly, it has no truly stable isotope. That makes lead (82) the heaviest element with stable isotopes.
Why technetium and promethium are radioactive
It seems odd that two elements in the middle of the table have no stable isotopes, surrounded by neighbours that do. It comes down to nuclear physics: for certain combinations of protons and neutrons, every possible isotope happens to be slightly less stable than a neighbouring nucleus it can decay into. Rules worked out by Josef Mattauch in the 1930s predict exactly this for elements 43 and 61.
- Technetium was the first element made artificially (1937) — its name comes from the Greek technetos, “artificial”. Its isotope technetium-99m is the most widely used radioisotope in medical imaging, used in tens of millions of scans every year.
- Promethium is the only lanthanide with no stable isotopes. It’s been used in some luminous paints and in tiny nuclear batteries.
Why heavy elements are all radioactive
In a nucleus, protons repel each other electrically, while the strong nuclear force holds all the nucleons together. The strong force only acts over very short distances, but electrical repulsion reaches across the whole nucleus. As nuclei get bigger, the repulsion between protons grows faster than the strong force can compensate. Beyond lead and bismuth, no arrangement of protons and neutrons is stable.
Heavy nuclei become more stable by shedding protons and neutrons, mostly through alpha decay or, for the heaviest, spontaneous fission. See alpha, beta and gamma radiation.
Radioactive elements you meet in everyday life
- Radon — a gas seeping from rocks and soil into buildings; the largest source of natural radiation exposure for most people.
- Potassium — not in the list above (it has stable isotopes), but about 0.012% of natural potassium is radioactive potassium-40. Your body contains a few thousand becquerels of it — and so do bananas, famously.
- Carbon — similarly, a trace of radioactive carbon-14 exists in every living thing. See carbon-14 dating.
- Americium — in ionization smoke detectors.
- Uranium and thorium — in rocks, granite countertops and some old glazes, at low levels.
- Tritium (hydrogen-3) — in glowing exit signs and watch dials.
Half-lives range enormously
Among the radioactive elements, the longest-lived isotopes range from billions of years to fractions of a second:
- Thorium-232: about 14 billion years
- Uranium-238: about 4.5 billion years
- Plutonium-244: about 80 million years
- Radium-226: about 1,600 years
- Polonium-209: about 125 years
- Francium-223: about 22 minutes
- Oganesson-294: under a millisecond
Thorium and uranium have survived since before the Earth formed; that’s why they’re found in nature. You can explore every isotope’s half-life on each element page, or calculate decay with the half-life calculator.
Quick answers
What is the lightest radioactive element? Technetium (43), if you mean elements with no stable isotopes. Hydrogen already has a radioactive isotope (tritium).
What is the heaviest stable element? Lead (82).
Are all radioactive elements dangerous? Radioactivity is dangerous at high doses, but the risk depends on the amount, the type of radiation, the half-life and whether it gets inside the body. Trace natural radioactivity is everywhere and unavoidable.
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