In normal fission reactors, the fuel used to start the nuclear fission is Uranium-235.
Generally, fuel rods enriched with uranium-235 are used to start the fission. When a nucleus of uranium-235 absorbs a neutron, it becomes unstable and then it breaks apart, producing two smaller nuclei, several neutrons and energy. The additional neutrons produced in the reaction are then absorbed by other nuclei of uranium-235, triggering other fission reactions, and so on.
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Atoms are basically tiny structures that make up everything. And a compound is something used in a scientific expirement. I don't get the question. You used improper grammar