What Are The Radioactive Elements? - Science Notes

This is a list of the radioactive elements of the periodic table. While all elements have some radioactive isotopes, only 37 elements have no stable isotopes. These elements are the “radioactive elements”.

Key Takeaways: Radioactive Elements

• Radioactive elements are those with no stable isotopes—there are 37 such elements on the periodic table.
• Technetium and promethium are the only radioactive elements lighter than bismuth.
• Most radioactive elements occur naturally or are formed in nuclear reactions and particle accelerators.
• Isotope stability varies widely—some decay in milliseconds, while others last millions of years.
• Radioactive isotopes have important uses in medicine, industry, science, and consumer products.
• Emission types include alpha, beta, gamma, or combinations of these, with different safety implications.

Radioactive Elements Resources

Download and print the radioactive elements periodic table and list of radioactive elements with their most stable isotope and half-life:

• Periodic Table: Color PNG | Black and White PDF
• List of Radioactive Elements: PNG | PDF

List of Radioactive Elements

This is a list of the 37 radioactive elements and their most stable isotopes. While many of these elements have half-lives so long they almost appear stable, the heavier elements decay almost instantly.

Why Are Some Elements Radioactive?

An element is radioactive when all of its isotopes are unstable, meaning their nuclei spontaneously decay over time into other elements. This instability results from an imbalance between the number of protons and neutrons or from a nucleus that’s simply too large to hold itself together.

Several factors contribute to radioactivity:

• Neutron-to-proton ratio: Stable nuclei tend to follow a certain ratio. Too many or too few neutrons cause instability.
• Large atomic nuclei: As atomic number increases, electrostatic repulsion between protons becomes stronger. Heavy elements like uranium or thorium can’t overcome this repulsion with the strong nuclear force alone, making them inherently unstable.
• Odd-even combinations: Nuclei with even numbers of protons and neutrons tend to be more stable. Isotopes with odd numbers of both are usually less stable.
• Shell effects and magic numbers: Some numbers of protons or neutrons confer extra stability (called “magic numbers”). Deviating from these can make isotopes more likely to decay.

The first 82 elements (through lead) all have at least one stable isotope, except for technetium (Z=43) and promethium (Z=61). All elements beyond lead are inherently unstable due to increasing nuclear size and repulsive forces.

Natural vs Synthetic Radioactive Elements

Radioactive elements can be either naturally occurring or synthetic. Natural radioactive elements come from cosmic or stellar processes and often have long half-lives. Synthetic elements are usually produced in reactors or accelerators and decay rapidly.

Natural Radioisotopes

Some natural radioactive elements form via nucleosynthesis in stars and supernovae. These primordial elements have long half-lives, so they existed before the Earth formed. Eventually, they decay into secondary radionuclides. Examples of primordial radioisotopes include thorium-232, uranium-238, and uranium-235, which decay into radium and polonium. However, some radioisotopes still form today. For example, cosmic radiation continuously produces carbon-14.

Synthetic Radioisotopes

There are a few ways of synthesizing radioactive elements. One involves placing an element in reactor and allowing neutrons to react and form products. An example of a radioactive element formed this way is iridium-192. Another process strikes a target with energetic particles. For example, fluorine-18 forms in particle accelerators. Sometimes researchers make a heavier element and obtain the desired product as part of the decay scheme. For example, technetium-99m comes from the decay of molybdenum-99. A technetium cow or moly cow is a device containing molybdenum-99, which has a half-life of 66 hours. Its decay product is technetium-99m, which only lasts about 6 hours. So, transporting the molybdenum isotopes allows delivery of the useful technetium isotope.

Fission Products

Nuclear fission occurs naturally in uranium or plutonium ore deposits, but most fission products come from nuclear power plants, nuclear tests, and thermonuclear weapons. For example, the radioactive fission products of uranium-235 include iodine, cesium, strontium, xenon, and barium isotopes.

Commercially Available Radionuclides

Some common isotopes are available (in small quantities) to researchers, medical professionals, industries, and even to the general public for element collections. For the most part, these radionuclides have relatively long half-lives, ranging from a few hours to several years. Most of these isotopes find use as tracers. The hydrogen isotope tritium is popular for glow in the dark items.

Gamma Emitters

• Barium-133
• Cadmium-109
• Cobalt-57
• Cobalt-60
• Europium-152
• Manganese-54
• Sodium-22
• Zinc-65
• Technetium-99m

Beta Emitters

• Strontium-90
• Thallium-204
• Carbon-14
• Tritium

Alpha Emitters

• Polonium-210
• Uranium-238

Emit Multiple Particles

• Cesium-137
• Americium-241

Frequently Asked Questions (FAQs)

Can radioactive elements ever become stable?

Yes, but only through nuclear decay. A radioactive element transforms into another element or isotope that may be stable or still radioactive. For example, uranium-238 eventually decays into stable lead-206 through a long decay chain. However, you cannot stabilize a radioactive element through physical or chemical means.

What is the most radioactive element?

The term “most radioactive” can mean the one with the shortest half-life or most intense emission. Among the elements, oganesson-294 has one of the shortest known half-lives (≈1.8 milliseconds). However, polonium-210 is considered one of the most intensely radioactive elements encountered outside particle accelerators, due to its high alpha particle emission rate.

Are all radioactive elements dangerous?

Not necessarily. The danger depends on:

• The type of radiation emitted (alpha, beta, gamma).
• The half-life (shorter-lived isotopes often release more energy per second).
• The route of exposure (ingested alpha emitters are more dangerous than external ones).

For example, tritium and americium are radioactive, but are relatively safe in low concentrations or enclosed devices.

How many radioactive elements are natural?

The number varies depending on how one defines “natural.” Around 34 radioactive elements occur in nature, including:

• Primordial radionuclides with long half-lives (e.g., uranium, thorium, potassium).
• Decay products of other natural radioisotopes (e.g., radium, polonium, radon).
• Cosmogenic isotopes produced by cosmic rays (e.g., carbon-14, tritium).

Other radioactive elements, like technetium and promethium, occur naturally in trace amounts due to spontaneous fission or decay chains but are more often synthesized.

What’s the difference between radioactive elements and radioactive isotopes?

• A radioactive isotope is an individual nuclide that decays over time, such as carbon-14 or cobalt-60.
• A radioactive element has no stable isotopes at all—every form of the element is radioactive. For example, every isotope of radon is radioactive, so radon is a radioactive element.

In contrast, elements like carbon or cobalt have both stable and radioactive isotopes and are not considered radioactive elements.

What is the most stable radioactive element?

That depends on the isotope. The most stable isotopes of radioactive elements have impressively long half-lives:

• Plutonium-244: ~80 million years
• Uranium-236: ~23.4 million years
• Neptunium-237: ~2.14 million years
• Technetium-98: ~4.2 million years

References

• International Atomic Energy Agency ENSDF database (2010).
• Loveland, W.; Morrissey, D.; Seaborg, G.T. (2006). Modern Nuclear Chemistry. Wiley-Interscience. ISBN 978-0-471-11532-8.
• Luig, H.; Kellerer, A. M.; Griebel, J. R. (2011). “Radionuclides, 1. Introduction”. Ullmann’s Encyclopedia of Industrial Chemistry. ISBN 978-3527306732. doi:10.1002/14356007.a22_499.pub2
• Martin, James (2006). Physics for Radiation Protection: A Handbook. ISBN 978-3527406111.
• Petrucci, R.H.; Harwood, W.S.; Herring, F.G. (2002). General Chemistry (8th ed.). Prentice-Hall.

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