Plutonium (Pu), a synthetic, radioactive element, is central to nuclear technology. Its density, a measure of mass per volume (g/cm³), is particularly notable.
The density of plutonium is not a single fixed value because the element exists in multiple solid forms called allotropes. The most common and stable form at room temperature, the alpha phase (α-Pu), exhibits an extremely high density of approximately 19.84 g/cm³. The full range of plutonium’s allotropes spans from about 16.00 to 19.86 g/cm³, a significant variation that makes the element metallurgically complex.
How Plutonium Compares to Other Heavy Elements
Common construction materials like steel have an average density of around 7.85 g/cm³, meaning plutonium (19.84 g/cm³) is more than twice as dense. Even lead, often associated with heaviness, has a density of only 11.34 g/cm³.
Plutonium is denser than both lead and gold (19.32 g/cm³). This makes plutonium one of the densest elements on the periodic table, rivaled only by a few others, such as the platinum-group metals. Depleted uranium, another heavy actinide, comes close with a density of about 19.1 g/cm³.
If one were to hold equal-sized blocks of lead and plutonium, the plutonium would feel nearly 75% heavier. This extreme packing efficiency is why the element is significant in specialized applications.
The Atomic Structure Behind Plutonium’s Weight
Plutonium’s high density originates from the sheer mass of its atoms and their efficient arrangement. The most common isotope, Plutonium-239, has a large nucleus containing 94 protons and 145 neutrons, giving it an atomic mass near 244 atomic mass units. This large number of particles contributes greatly to the element’s overall mass per atom.
The arrangement of these massive atoms into a solid structure is governed by plutonium’s unique electron configuration, which involves the complex behavior of its 5f-shell electrons. These electrons are positioned at a transition point where they are neither fully localized nor fully shared, leading to unusual and complex metallic bonding. This electronic complexity is the reason plutonium exhibits six or more different solid-state crystal structures, or allotropes, at relatively low temperatures.
The alpha phase, stable at room temperature, has a low-symmetry monoclinic crystal structure that facilitates a very tight packing of the atoms. This structure accounts for its maximum density of 19.84 g/cm³. Other phases, like the delta phase, have a less efficient cubic packing arrangement and a significantly lower density, closer to 15.9 g/cm³.
Practical Implications of High Density
Plutonium’s extreme density has profound consequences for its practical application, particularly in nuclear devices. The most direct consequence is its relationship to the concept of critical mass. Critical mass is the minimum amount of fissile material needed to sustain a nuclear chain reaction.
A higher density means the atoms are closer together, increasing the probability that a neutron released during fission will hit another nucleus and continue the reaction. Because the critical mass decreases significantly as the density increases, a small sphere of highly dense plutonium can achieve criticality where a larger sphere of a less dense material could not. For example, the critical mass for uncompressed, pure Plutonium-239 is around 10 kilograms.
In modern nuclear applications, explosive charges are used to rapidly compress a sub-critical mass of plutonium to a much higher density. This compression forces the material into a supercritical state, initiating the powerful energy release. Plutonium’s high density also plays a role in shielding and storage, as the material’s sheer mass is a factor in designing containment vessels and counterweights.