If the nucleus of a uranium atom is hit with a neutron and it splits into two smaller atoms and three more neutrons, then has occurred.

Nuclear fission is a reaction in which the nucleus of atom when bombarded with a sub-atomic particle splits into smaller nuclei along with the release of by-product particles and large amounts of heat

The given scenario describes a nuclear fission reaction. If we consider the U-235 being bombarded by neutron then we have:

₉₂U²³⁵ + ₀n¹ → ₅₆Ba¹⁴⁴ +   ₃₆Kr⁸⁹  + 3₀n¹  + heat

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