What specific property of Uranium-235 makes it suitable for use in nuclear weapons?

The specific property of Uranium-235 (U-235) that makes it suitable for use in nuclear weapons is its high probability of undergoing nuclear fission when struck by a neutron, particularly a slow or 'thermal' neutron, and the fact that each fission event releases multiple neutrons. This ability to readily fission and release neutrons is crucial for sustaining a rapidly escalating nuclear chain reaction. When a neutron strikes a U-235 nucleus, the nucleus splits into two smaller nuclei (fission fragments) and releases an average of 2 to 3 additional neutrons, along with a significant amount of energy. These released neutrons can then strike other U-235 nuclei, causing them to fission and release even more neutrons, creating a self-sustaining chain reaction. The energy released from these fission events is what creates the explosive power of a nuclear weapon. The 'critical mass' of U-235 (the amount needed to sustain a chain reaction) is also relatively manageable compared to other isotopes, making it practical for weapon design. While Uranium-238 is more abundant, it primarily undergoes fission only with high energy (fast) neutrons, and tends to absorb slow neutrons, inhibiting a chain reaction, making it unsuitable for initiating a rapid, uncontrolled chain reaction in a weapon. Therefore, the combination of high fission probability with thermal neutrons and the release of multiple neutrons per fission makes U-235 ideal for nuclear weapons.