Being radioactive, uranium is not a stable element.
Uranium slowly decay to form the stable nucleus of Lead after a series of nuclear reactions.
It becomes most stable when its nucleus is filled, not when it is filling it.
Uranium 235 is unstable because it is a radioactive isotope. This means that it is constantly decaying and emitting radiation. The reason it is unstable is because it has too many neutrons in its nucleus. The neutron is a unstable particle, and when there are too many of them in one place, they can cause problems. When uranium 235 decays, it emits alpha particles, which are high-energy particles that can damage DNA and cause cancer.
Uranium hasn't stable isotopes.
Thorium, radium, radon, polonium, thallium, etc.
Nuclear decay.
The nucleus would become unstable because you need a certain amount of neutrons, electrons, and protons for it to be stable.
92 protons in uranium nucleus
The nucleus is too large to be stable. There is the theory of grouping of nucleons into alpha particles inside the nucleus and, through oscillations of the nucleus, one of these on one end of the nucleus can be repelled with a great enough force to push it out of the nucleus.
Krypton (isotopes 83 to 86) and barium (isotopes 138 and 139) are fission products of uranium, resulting from the nuclear fission of uranium atom nucleus.
Nuclear decay.
Uranium has 92 protons.