As stated before, Uranium-235 undergoes fission reactions when bombarded with neutrons. The Uranium atom then splits into two new atoms, and releases a further neutron (continuing the chain reaction). This reactions can be written in a familiar 'chemical equation' form.
1n + 235U → 140Ba + 93Kr + 31n
This equation shows that when one neutron strikes a Uranium-235 atom its splits it into a Barium isotope of relative mass 140, and a Krypton isotope of relative mass 93, as well as releasing 3 neutrons.
The total mass ejected (236) is equal to the total amount put in (236). This reaction is therefore legitimate because both sides of the equation balance.
Plutonium-239 is the most common isotope of Plutonium and is fissable. It undergoes fission in the same way as Uranium-235, releasing neutrons to sustain a chain reaction if left unchecked.
239Pu + 1n → 140Ba + 90Sr + 31n
The Nuclear fusion between Hydrogen isotopes is the same reaction which occurs on the surface of the sun. The reaction doesn't simply happen when two isotopes are close to each other. The reaction is triggered when the gaseous isotopes are heated to extreme degrees under great pressure. The equation for these reactions are shown below.
2H + 2H → 3He + 1n
2H + 3H → 4He + 1n
Tritium has a very short half-life, and for the purposes of nuclear weaponary, is essentially useless. However, scientists overcame this problem by using Lithium, after realising that when Lithium-6 undergoes fision it creates Tritium and Helium-4. Lithium-7 however also ejects a neutron as well as Helium and Tritium.
It was also noted that the main form of radiation released by the fision of Lithium was X-ray radiation. X-ray radiation can also provide the extreme temperatures required to initiate Hydrogen fusion!
For this reason, in Hydrogen nuclear weapons Deutirium is combined with Lithium to make the reaction even more powerful.
It is worth noting that as of yet nobody has been able to control nuclear fusion, however efforts are underway to do this, and the first steps have already been taken at Princeton University.
