Uranium-236

U-235 is highly unstable which makes it highly fissionable or splittable.

When uranium U-235 is slammed by a neutron, it becomes uranium-236.

U-236 is so unstable that it almost immediately splits into two more stable atoms, Krypton and Barium.

In the process of splitting and creating two more stable atoms, a whole bunch of energy is released along with three more neutrons.

These three neutrons fly out and slam more U-235 atoms, and thus a chain reaction occurs causing more and more U-235 to be split and ultimately causes a huge explosion.