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In nuclear physics, an element's critical mass refers to the smallest quantity of fissile material needed in order to sustain a nuclear chain reaction, such as the controlled reactions observed in nuclear reactors or the uncontrolled reactions used in nuclear weapons. Critical masses are subject to various factors, such as its measurements, density, and chemical composition. A sphere is considered the optimum shape for fission, as it will allow the largest number of reactions using the smallest or lightest amount of material.

Uranium

Uranium-235 is the only naturally-occurring fissile material found on earth, but it makes up just 0.72 percent of all naturally occurring uranium, while almost all the rest is made up of uranium-238. For this reason, uranium can be enriched by separating these isotopes and then creating new compositions with a higher share of ²³⁵U. A concentration of 20 percent or more of ²³⁵U is considered "highly enriched" or "weapons grade", although most nuclear weapons using this material had a concentration of 80 percent or more. Hypothetically, a sphere of 100 percent uranium-235 would have a critical mass of 49 kilograms.

Synthetic elements

Unlike uranium, the rest of the elements on this list either do not occur naturally on earth, or occur in such small trace amounts that it is almost impossible to collect. Because of this, these elements must be synthetically created using a nuclear reactor or in a particle accelerator, or they may be a by-product of an atomic explosion.