Which Elements Are Radioactive? A Plain-English Guide to Unstable Atoms

38 of the 118 elements have no stable form: technetium, promethium and everything from bismuth up. See why atoms decay and what half-life really means.

The elements with no stable form at all, so that every atom of them is radioactive, are technetium (element 43), promethium (61) and every element from bismuth (83) up to oganesson (118). That makes 38, or 37 if you treat bismuth as stable (more on that below). The other 80 elements (81 with bismuth) each have at least one version that has never been seen to decay.

The careful phrase is "no stable isotope", because radioactivity belongs to isotopes rather than to whole elements. Isotopes are versions of one element with the same number of protons but different numbers of neutrons (protons and neutrons are the two kinds of particle in the atom's central nucleus; the atom article has the full picture). Carbon-12 lasts forever; carbon-14, with two extra neutrons, slowly changes. Both are carbon, and the number in each name counts the protons and neutrons together. An isotope is stable if it never changes by itself, and radioactive when its nucleus will, at some unpredictable moment, decay by firing out a particle or a burst of energy. Many ordinary elements have a few radioactive isotopes too.

Everything after lead, plus two gaps

Walk along the table in order of atomic number (the number of protons) and almost everything is stable until you reach lead, element 82. After lead, no element is stable (bismuth is the borderline case, below): elements 83 to 118, thirty-six of them, to the end of the table. The two gaps come earlier. Technetium sits in the middle of the table, directly beneath manganese. Promethium sits in the lanthanide row (the detached row below the main table), between neodymium and samarium. Their next-door neighbours all have stable isotopes, which is why they look like gaps. The full rule is:

• technetium (43): 1 element
• promethium (61): 1 element
• bismuth (83) to oganesson (118): 36 elements

That adds up to 1 + 1 + 36 = 38.

The bismuth footnote. Bismuth-209 does decay, but its half-life (the time for half a sample to go; more on that shortly) is about 2 × 10¹⁹ years, a 2 followed by nineteen zeros. The universe is about 13.8 billion years old, so that is over a billion times longer. In a whole kilogram of bismuth, roughly one atom decays every five minutes, and the decay was first detected only in 2003. For daily life bismuth is stable, which is why you will see both 80 and 81 given as the number of stable elements.

Try it on Periodic Bubbles. Open the bubbles and find the two gaps, technetium and promethium. Their atomic masses are whole numbers, 98 and 145, because neither has a steady natural mix of isotopes to average, so the table shows the mass number (protons plus neutrons) of one longer-lived isotope instead. Bismuth (208.98) and uranium (238.03) show decimals because they do occur naturally in steady amounts. Bismuth has a single natural isotope, so its decimal is the measured mass of that one isotope, not an average, while uranium's is an average of its isotopes (the atomic mass article explains averaging). Then try the Daily Bubble: guess an actinide (one of the heavy metals in the bottom row under the table, from actinium to lawrencium) and the line under the board calls the whole family radioactive.

Why do some atoms fall apart?

Radioactivity is a story about the nucleus. Protons carry positive charge, so they push each other apart. A second force, the strong nuclear force, sticks protons and neutrons together, but only between close neighbours. Picture a huddle of people who dislike each other's company, held in place by tape that only works between people shoulder to shoulder. Neutrons carry no charge, so they add tape without adding push. (A simplification: real nuclei follow quantum rules, not tape.) The balance of neutrons and protons needs one more idea: each kind fills its own energy levels, like seats in a theatre, so a lopsided nucleus can settle into a lower-energy arrangement, one that holds together more comfortably, by turning one kind into the other. Together these give three ways to be unstable.

• Too many neutrons. Carbon-14 has 6 protons and 8 neutrons. One neutron turns into a proton, an electron (a tiny, negatively charged particle) flies out, and the atom becomes nitrogen-14.
• Too few neutrons. The reverse happens: a proton turns into a neutron, and the atom becomes the element with one proton fewer.
• Simply too big. The push reaches across the whole huddle, but the tape reaches only the neighbours. Heavy nuclei need extra neutrons as filler (lead-208 has 82 protons and 126 neutrons), yet there is a limit. Lead is the last element with stable isotopes, and lead-208 is the heaviest nucleus never seen to decay.

Technetium and promethium are mid-sized, so size is not their problem. Their trouble is a rule of thumb: a nucleus stays put only if no neighbouring element could hold the same total of protons and neutrons more tightly. Protons like to come in pairs, and so do neutrons, so an element with an odd number of protons, like these two (43 and 61), is often beaten by a neighbour with an even number. For technetium and promethium a neighbour always wins: whatever the number of neutrons, the same total is held more tightly in molybdenum or ruthenium for technetium, and neodymium or samarium for promethium, so there is always somewhere better to go. An odd number is not fatal in itself: sodium has 11 protons and a stable form.

What is half-life?

Nobody can say when one particular unstable atom will decay, much as nobody can call a single coin toss. A large crowd is dependable, though. Toss 64 coins and set aside every one that lands heads, as if those atoms had decayed. About 32 are left. Toss those and remove the heads again: about 16. Each round halves the crowd, and one round is one half-life.

So half-life is the time it takes half the unstable nuclei in a sample to decay. Equivalently, it is the time for the sample's activity (its count rate on a radiation detector) to fall to half. It is not half the sample's life: after two half-lives a quarter is left, not nothing. Real atoms do not toss coins, but each has a fixed chance of decaying in any given second, however long it has already waited, which is why the halving is so regular.

A worked example uses technetium-99m, with a half-life of about 6 hours (the m means the nucleus holds spare energy it will soon release). Start with 800,000 atoms. After 6 hours about 400,000 remain, after 12 hours 200,000, after 18 hours 100,000, and after a day 50,000, one sixteenth of the start. Real half-lives span an enormous range:

For the same number of atoms, a short half-life means intense but brief, a long one weaker but lasting.

Alpha, beta and gamma in brief

• Alpha: two protons and two neutrons thrown out as one lump (a helium nucleus). The atomic number falls by 2 and the mass number (protons plus neutrons) by 4: uranium-238 becomes thorium-234. A sheet of paper stops it.
• Beta: a neutron turns into a proton and flings out a fast electron, so the atomic number rises by 1 and the mass number is unchanged: carbon-14 becomes nitrogen-14. (The reverse kind, a proton becoming a neutron, lowers it by 1.) A few millimetres of aluminium stops it.
• Gamma: a burst of very high-energy electromagnetic radiation, a relative of X-rays and light, which leaves the element unchanged. Thick lead or concrete cuts it down a lot, though nothing stops it completely.

Does radioactive mean dangerous?

Not by itself. "Radioactive" describes what a nucleus does, not how much it matters. What counts is how many atoms, what kind of radiation, and how close you are for how long. Strong sources such as plutonium are seriously dangerous and tightly controlled, but you live among tiny amounts of radioactive atoms and are partly made of them.

Bananas, and you: potassium-40. About 0.012 per cent of natural potassium is potassium-40, with a half-life of about 1.25 billion years. Every banana, potato and person carries a little, and the body keeps its potassium level steady, so a banana does not pile up.

Every living thing: carbon-14. Cosmic rays (fast particles from space) strike the upper atmosphere and turn a little nitrogen into carbon-14, about one carbon atom in a million million. Plants take it in with carbon dioxide and animals eat the plants, so living things carry it and keep topping up. Once something dies the topping up stops and the carbon-14 halves about every 5,700 years (the atom article shows how radiocarbon dating uses this).

Radon in some UK homes. Radon is a radioactive noble gas (a gas that hardly reacts with anything) made when radium in rock and soil decays (see radium versus radon). It can seep into buildings, and breathing it in, with the radioactive solids it turns into, raises the risk of lung cancer over many years, so testing is advised. Levels vary hugely by area, with granite areas such as Cornwall and Devon among the highest, though other rocks can be high too. The UK Health Security Agency (UKHSA) publishes the official radon maps and testing guidance at UKradon.org, the place to check a particular address.

Smoke alarms: americium. Ionisation smoke alarms, the kind that sense smoke by how it disturbs a tiny electric current in the air, contain a speck of americium-241, less than a millionth of a gram, sealed in a small metal chamber. Its alpha particles are what make that current flow (the lanthanides and actinides article shows how), and the casing holds them in. Many newer alarms use light instead and contain no americium.

Hospital scans: technetium-99m. The short half-life is the point: the small radioactive dose given to a patient does its job and is mostly gone within a day. The elements in medicine covers how the scans work.

Which radioactive elements are natural, and which are made?

There are three groups.

• Survivors. Uranium and thorium have half-lives so long that plenty of atoms from the Earth's formation remain, and bismuth lasts longer still.
• Decay-chain links. Radium, polonium, radon, actinium and protactinium are made as uranium and thorium decay (a decay chain is a series of steps, each atom turning into the next element) and lost as fast as they form. Astatine and francium are the same: estimates put each at a few tens of grams at most in the Earth's crust at any one time, astatine's usually far less.
• Made by people. Technetium, promethium, neptunium and plutonium come mainly from reactors (only traces occur in uranium ore), and every element after plutonium from reactors or particle accelerators (how new elements are made), the last ones lasting fractions of a second.

The actinide row, from actinium to lawrencium, takes in natural thorium and uranium, the decay-chain links actinium and protactinium, and elements that people make, and the lanthanides and actinides article tells that story.

Key takeaways

• 38 elements have no stable isotope (37 if bismuth counts as stable): technetium (43), promethium (61) and bismuth (83) to oganesson (118).
• Radioactivity belongs to isotopes, so "no stable isotope" is the careful phrase for an element.
• Nuclei decay when the neutron balance is wrong or they are too big; lead is the last element with a truly stable isotope.
• Half-life is the time for half the unstable nuclei in a sample to decay, from fractions of a second to billions of years and beyond.
• Radioactive does not automatically mean dangerous: bananas, people and some smoke alarms contain radioactive atoms, but strong sources need careful handling.