Why Uranium and Plutonium?
Scientists knew that the most common isotope, uranium 238, was not suitable for a nuclear weapon. There is a fairly high probability that an incident neutron would be captured to form uranium 239 instead of causing a fission. However, uranium 235 has a high fission probability.
Of natural uranium, only 0.7% is uranium 235. This meant that a large amount of uranium was needed to obtain the necessary quantities of uranium 235. Also, uranium 235 cannot be separated chemically from uranium 238, since the isotopes are chemically similar.
Alternative methods had to be developed to separate the isotopes. This was another problem for the Manhattan Project scientists to solve before a bomb could be built.
Research had also predicted that plutonium 239 would have a high fission probability. However, plutonium 239 is not a naturally occurring element and would have to be made. The reactors at Hanford, Washington were built to produce plutonium.