Plutonium (₉₄Pu) is an artificial element, except for trace quantities resulting from neutron capture by uranium, and thus a standard atomic weight cannot be given. Like all artificial elements, it has no stable isotopes. It was synthesized long before being found in nature, the first isotope synthesized being ²³⁸Pu in 1940. Twenty-two plutonium radioisotopes have been characterized. The most stable are ²⁴⁴Pu with a half-life of 80.8 million years; ²⁴²Pu with a half-life of 373,300 years; and ²³⁹Pu with a half-life of 24,110 years; and ²⁴⁰Pu with a half-life of 6,560 years. This element also has eight meta states; all have half-lives of less than one second.

The known isotopes of plutonium range from ²²⁶Pu to ²⁴⁷Pu. The primary decay modes before the most stable isotope, ²⁴⁴Pu, are spontaneous fission and alpha decay; the primary mode after is beta emission. The primary decay products before ²⁴⁴Pu are isotopes of uranium and neptunium (not considering fission products), and the primary decay products after are isotopes of americium.

• Plutonium-238 has a half-life of 87.74 years^[12] and emits alpha particles. Pure ²³⁸Pu for radioisotope thermoelectric generators that power some spacecraft is produced by neutron capture on neptunium-237 but plutonium from spent nuclear fuel can contain as much as a few percent ²³⁸Pu, originating from ²³⁷Np, alpha decay of ²⁴²Cm, or (n,2n) reactions.
• Plutonium-239 has half-life 24,100 years. ²³⁹Pu and ²⁴¹Pu are fissile; meaning their nuclei can split by being bombarded by slow thermal neutrons, releasing energy, gamma radiation and more neutrons. It can therefore sustain a nuclear chain reaction, leading to applications in nuclear weapons and nuclear reactors. ²³⁹Pu is synthesized by irradiating uranium-238 with neutrons in a nuclear reactor, then recovered via nuclear reprocessing of the fuel. Further neutron capture produces successively heavier isotopes.
• Plutonium-240 has a high rate of spontaneous fission, raising the background neutron radiation of plutonium. Plutonium is graded by proportion of ²⁴⁰Pu: weapons grade (<7%), fuel grade (7–19%) and reactor grade (>19%). Lower grades are less suited for bombs and thermal reactors but can fuel fast reactors.
• Plutonium-241 is fissile, but beta decays with a half-life of 14 years to americium-241.
• Plutonium-242 is not fissile, nor very fertile (requiring 3 more neutron captures to become fissile); and has a low neutron capture cross section, and a longer half-life than any of the lighter isotopes.
• Plutonium-244 is the most stable isotope of plutonium, with a half-life of about 80 million years. It is not significantly produced in nuclear reactors because ²⁴³Pu has a short half-life, but some is produced in nuclear explosions. ²⁴⁴Pu has been found in interstellar space^[13] and has the second longest half-life of any non-primordial radioisotope.

²³⁹Pu, a fissile isotope that is the second most used nuclear fuel in nuclear reactors after uranium-235, and the most used fuel in the fission portion of nuclear weapons, is produced from uranium-238 by neutron capture followed by two beta decays.

²⁴⁰Pu, ²⁴¹Pu, and ²⁴²Pu are produced by further neutron capture. The odd-mass isotopes ²³⁹Pu and ²⁴¹Pu have about a 3/4 chance of undergoing fission on capture of a thermal neutron and about a 1/4 chance of retaining the neutron and becoming the next heavier isotope. The even-mass isotopes are fertile but not fissile and also have a lower probability (cross section) of neutron capture; therefore, they tend to accumulate in nuclear fuel used in a thermal reactor, the design of nearly all nuclear power plants today. In plutonium that has been used a second time in thermal reactors in MOX fuel, ²⁴⁰Pu may even be the most common isotope. All plutonium isotopes and other actinides, however, are fissionable with fast neutrons. ²⁴⁰Pu does have a moderate thermal neutron absorption cross section, so that ²⁴¹Pu production in a thermal reactor becomes a significant fraction as large as ²³⁹Pu production.

²⁴¹Pu has a half-life of 14 years, and has slightly higher thermal neutron cross sections than ²³⁹Pu for both fission and absorption. While nuclear fuel is being used in a reactor, a ²⁴¹Pu nucleus is much more likely to fission or to capture a neutron than to decay. ²⁴¹Pu accounts for a significant portion of fissions in thermal reactor fuel that has been used for some time. However, in spent nuclear fuel that does not quickly undergo nuclear reprocessing but instead is cooled for years after use, much or most of the ²⁴¹Pu will beta decay to americium-241, one of the minor actinides, a strong alpha emitter, and difficult to use in thermal reactors.

²⁴²Pu has a particularly low cross section for thermal neutron capture; and it takes three neutron absorptions to become another fissile isotope (either curium-245 or ²⁴¹Pu) and fission. Even then, there is a chance either of those two fissile isotopes will fail to fission but instead absorb a fourth neutron, becoming curium-246 (on the way to even heavier actinides like californium, which is a neutron emitter by spontaneous fission and difficult to handle) or becoming ²⁴²Pu again; so the mean number of neutrons absorbed before fission is even higher than 3. Therefore, ²⁴²Pu is particularly unsuited to recycling in a thermal reactor and would be better used in a fast reactor where it can be fissioned directly. However, ²⁴²Pu's low cross section means that relatively little of it will be transmuted during one cycle in a thermal reactor. ²⁴²Pu's half-life is about 15 times as long as ²³⁹Pu's half-life; therefore, it is 1/15 as radioactive and not one of the larger contributors to nuclear waste radioactivity. ²⁴²Pu's gamma ray emissions are also weaker than those of the other isotopes.^[15]

²⁴³Pu has a half-life of only 5 hours, beta decaying to americium-243. Because ²⁴³Pu has little opportunity to capture an additional neutron before decay, the nuclear fuel cycle does not produce the long-lived ²⁴⁴Pu in significant quantity.

²³⁸Pu is not normally produced in as large quantity by the nuclear fuel cycle, but some is produced from neptunium-237 by neutron capture (this reaction can also be used with purified neptunium to produce ²³⁸Pu relatively free of other plutonium isotopes for use in radioisotope thermoelectric generators), by the (n,2n) reaction of fast neutrons on ²³⁹Pu, or by alpha decay of curium-242, which is produced by neutron capture of ²⁴¹Am. It has significant thermal neutron cross section for fission, but is more likely to capture a neutron and become ²³⁹Pu.

The fission cross section for ²³⁹Pu is 747.9 barns for thermal neutrons, while the activation cross section is 270.7 barns (the ratio approximates to 11 fissions for every 4 neutron captures). The higher plutonium isotopes are created when the uranium fuel is used for a long time. For high burnup used fuel, the concentrations of the higher plutonium isotopes will be higher than the low burnup fuel that is reprocessed to obtain weapons grade plutonium.

²³⁹Pu is one of the three fissile materials used for the production of nuclear weapons and in some nuclear reactors as a source of energy. The other fissile materials are uranium-235 and uranium-233. ²³⁹Pu is virtually nonexistent in nature. It is made by bombarding uranium-238 with neutrons. Uranium-238 is present in quantity in most reactor fuel; hence ²³⁹Pu is continuously made in these reactors. Since ²³⁹Pu can itself be split by neutrons to release energy, ²³⁹Pu provides a portion of the energy generation in a nuclear reactor.

There are small amounts of ²³⁸Pu in the plutonium from usual reactors. However, isotopic separation would be quite expensive compared to another method: when ²³⁵U captures a neutron, it is converted to an excited state of ²³⁶U. Some of the excited ²³⁶U nuclei undergo fission, but some decay to the ground state of ²³⁶U by emitting gamma radiation. Further neutron capture creates ²³⁷U; which, with a half-life of 7 days, decays to ²³⁷Np. Since nearly all neptunium is produced in this way or consists of isotopes that decay quickly, one gets nearly pure ²³⁷Np. After chemical separation of neptunium, ²³⁷Np is again irradiated by reactor neutrons to be converted to ²³⁸Np, which decays to ²³⁸Pu with a half-life of 2 days.

²⁴⁰Pu undergoes spontaneous fission at a small but significant rate (5.8×10⁻⁶%).^[1] The presence of ²⁴⁰Pu limits the plutonium's use in a nuclear bomb, because a neutron from spontaneous fission starts the chain reaction prematurely, causing an early release of energy that disperses the core before full implosion is reached. This prevents most of the core from participation in the chain reaction and reduces the bomb's yield.

Plutonium consisting of more than about 90% ²³⁹Pu is called weapons-grade plutonium; plutonium from spent nuclear fuel from commercial power reactors generally contains at least 20% ²⁴⁰Pu and is called reactor-grade plutonium. However, modern nuclear weapons use fusion boosting, which mitigates the predetonation problem; if the pit can generate a nuclear weapon yield of even a fraction of a kiloton, which is enough to start deuterium–tritium fusion, the resulting burst of neutrons will fission enough plutonium to ensure a yield of tens of kilotons.

Contamination due to ²⁴⁰Pu is the reason plutonium weapons must use the implosion method. Theoretically, pure ²³⁹Pu could be used in a gun-type bomb, but achieving this level of purity is prohibitively difficult. ²⁴⁰Pu contamination has proven a mixed blessing. While it created delays and headaches during the Manhattan Project because of the need to develop implosion technology, those same difficulties are a barrier to nuclear proliferation. Implosion bombs are also inherently more efficient and less prone to accidental detonation than are gun-type bombs.

• Isotope masses from:
  • Audi, Georges; Bersillon, Olivier; Blachot, Jean; Wapstra, Aaldert Hendrik (2003), "The NUBASE evaluation of nuclear and decay properties", Nuclear Physics A, 729: 3–128, Bibcode:2003NuPhA.729....3A, doi:10.1016/j.nuclphysa.2003.11.001
• Half-life, spin, and isomer data selected from the following sources.
  • Audi, Georges; Bersillon, Olivier; Blachot, Jean; Wapstra, Aaldert Hendrik (2003), "The NUBASE evaluation of nuclear and decay properties", Nuclear Physics A, 729: 3–128, Bibcode:2003NuPhA.729....3A, doi:10.1016/j.nuclphysa.2003.11.001
  • National Nuclear Data Center. "NuDat 2.x database". Brookhaven National Laboratory.
  • Holden, Norman E. (2004). "11. Table of the Isotopes". In Lide, David R. (ed.). CRC Handbook of Chemistry and Physics (85th ed.). Boca Raton, Florida: CRC Press. ISBN 978-0-8493-0485-9.

• Magurno, B. A.; Pearlstein, S., eds. (1981). Proceedings of the conference on nuclear data evaluation methods and procedures. BNL-NCS 51363 (PDF). Vol. II. Upton: Brookhaven National Laboratory. Archived (PDF) from the original on March 8, 2021. Retrieved August 6, 2014.
• Miner, William N.; Schonfeld, Fred W. (1968). "Plutonium". In Clifford A. Hampel (ed.). The Encyclopedia of the Chemical Elements. New York (NY): Reinhold Book Corporation. pp. 540–546. LCCN 68029938.