Nuclear fission
For a few isotopes (an atom with the same amount of protons but a different amount of neutrons) such fission can release many neutrons.  If those neutrons then hit other atoms, they will make the other atoms split. This can happen again and again. This is called a nuclear chain-reaction, and it can release huge amounts of energy. Nuclear chain-reactions occur most commonly with Uranium-235. The splitting of Uranium-235 is jumpstarted when a neutron is slammed into its positively charged nucleus. This results in two lighter isotopes (Barium-141 and Krypton-92), whose total mass is less than that of the original Uranium isotope. The “mass defect” is converted into various forms of energy. A neutron is also released, triggering the afore-mentioned chain reaction as each additional neutron causes further collisions with existing Uranium isotopes in the environment.