Final answer:
Uranium and plutonium are both used in atomic bombs, but they have different characteristics. Plutonium is easier to fission with neutrons and has a smaller critical mass, making it more practical for use in fission bombs. Uranium bombs are developed simultaneously due to uncertainties in plutonium availability.
Step-by-step explanation:
Uranium and plutonium are both used in atomic bombs, but they have different characteristics and properties. Plutonium is recognized as a superior fission material because it is easier to fission with neutrons. It has a smaller critical mass and can be used in smaller quantities compared to uranium. This makes it more practical for use in fission bombs. On the other hand, uranium bombs are developed simultaneously due to uncertainties in plutonium availability. The critical mass in a uranium bomb is held together by explosive charges, while the critical mass in plutonium bombs can be held together with a relatively simple device. These differences in properties and availability have led to the use of both uranium and plutonium in atomic bombs.