Plutonium (Pu) is a heavy, radioactive, and synthetic element with an atomic number of 94. Its existence is almost entirely due to human-made nuclear processes, distinguishing it from most other elements on the periodic table. Unlike elements with a stable presence in nature, plutonium’s standard atomic weight cannot be determined by averaging naturally occurring isotopic abundances. The accepted mass is based on its longest-lived isotope, Plutonium-244, and is generally listed in atomic mass units (u).
Defining Atomic Mass and the Standard Atomic Weight of Plutonium
The concept of atomic mass is distinct from the mass number, which is a common point of confusion. The mass number is simply the count of protons and neutrons within the nucleus of a single atom, always resulting in a whole number. Atomic mass, or atomic weight, is the calculated weighted average of the masses of all naturally occurring isotopes of an element. This average typically appears on the periodic table as a decimal value, accounting for the relative abundance of each isotope.
Plutonium’s mass is often presented in square brackets to signify that it is a synthetic element lacking a consistent natural isotopic abundance. Because its isotopes are produced artificially and are all radioactive, the standard atomic weight must be a calculated value. This reference value is taken from the most stable isotope, Plutonium-244, which has a precise isotopic mass of 244.0642044 u and a half-life of over 80 million years.
While Plutonium-244 provides the official reference mass, the mass of other isotopes is often more relevant in practical nuclear science. Plutonium-239, for instance, is the isotope most commonly produced and utilized in nuclear applications. Its specific mass, 239.052162 u, directly influences calculations for nuclear fuel and weapons design. Understanding the mass number of each isotope is therefore more informative than relying on the bracketed standard atomic weight.
Key Plutonium Isotopes and Their Distinct Mass Numbers
Plutonium exists in many isotopic forms, but a few are important due to their specific nuclear properties. Plutonium-239, with 94 protons and 145 neutrons, is the most significant isotope because it is highly fissile, meaning it can easily sustain a nuclear chain reaction. This isotope has a half-life of approximately 24,110 years, making it suitable for use in nuclear reactors and weapons.
Plutonium-238 has a mass number of 238 and a much shorter half-life of 87.7 years. This isotope is a powerful alpha-particle emitter and is non-fissile, generating significant decay heat, about 0.57 watts per gram. This thermal output makes it the fuel for Radioisotope Thermoelectric Generators (RTGs), which provide long-duration electrical power for deep space missions.
Plutonium-240 is a common isotope in manufactured plutonium, created when Pu-239 absorbs a neutron but does not immediately fission. With a half-life of 6,561 years, this isotope has a high rate of spontaneous fission, releasing a continuous stream of neutrons. This characteristic makes Pu-240 an undesirable contaminant in weapons-grade plutonium. Its neutron emission can cause a premature, low-yield detonation known as a “fizzle,” so its concentration must be kept below seven percent for weapons applications.
How Plutonium is Synthesized
Plutonium is a transuranic element, meaning its atomic number is greater than that of Uranium. It is primarily created artificially in nuclear reactors. The process begins with Uranium-238, the most common isotope of uranium, which is considered a fertile material. When U-238 is exposed to the neutron flux within a reactor core, it captures a neutron.
This neutron capture transforms the U-238 nucleus into the highly unstable isotope Uranium-239. U-239 has an extremely short half-life of about 23.5 minutes and immediately begins beta decay. During this decay, a neutron converts into a proton, emitting an electron. This raises the atomic number by one, transforming the element into Neptunium-239 (Np-239).
Neptunium-239 is also unstable and undergoes a second beta decay, with a half-life of about 2.36 days. This second transformation converts another neutron into a proton, increasing the atomic number to 94 and completing the transmutation into Plutonium-239. This multi-step process requires Plutonium to be chemically separated from the spent nuclear fuel after irradiation.
Primary Applications of Plutonium
The unique nuclear properties of plutonium isotopes have led to several distinct applications. The most widely known application is in nuclear weapons, which rely on the fissile nature of Plutonium-239. The high energy density and low critical mass of Pu-239 allow for the creation of compact implosion-type nuclear devices.
Plutonium-239 is also a valuable fuel source for the nuclear power industry. It is typically blended with uranium oxide to create Mixed Oxide (MOX) fuel. Using MOX fuel allows the plutonium generated as a byproduct in traditional reactors to be recycled, extracting more energy from the original uranium and reducing the volume of long-lived nuclear waste.
Beyond terrestrial power generation, Plutonium-238 plays a unique role in space exploration. The heat generated by its alpha decay is converted into electricity by Radioisotope Thermoelectric Generators (RTGs). These reliable power sources have been used for decades to power spacecraft venturing into the outer solar system, such as the Voyager probes and the Curiosity and Perseverance Mars rovers, where solar power is ineffective.