what element was used in the hiroshima bomb code example

Example: The Hiroshima bomb

The atomic bomb that hit Hiroshima and Nagasaki was launched in Aug 6, 1945
and August 9, 1945 respectively. It was released by the US Military. it was an 
experiment done by a few scientists to test the potential of splitting an atom 
into 2 parts. The bomb was used as a one-time use to make Japan surrender and 
end the war once and for all. But the outcome had resulted in a total of an 
estimated 120,000+ deaths (including both the Hiroshima and Nagasaki explosion).