When an atom's nucleus is split apart, the process is known as nuclear fission. When this happens a great amount of energy is released as light and neutrons. One way an atom can be split is by stripping off its electrons with charged plates or light pulses and sending it through a curved magnetic field in a particle accelerator.
Knowledge of the basic anatomy of an atom is essential to understanding what happens when an atom is split. An atom has a nucleus that is its central core, comprised of neutrons and protons. When you add the number of protons and neutrons together, you get the atomic weight of an element. The number of protons in the nucleus determines the atomic number. In some instances, atoms of the same element may have the same atomic number, but differ in their atomic weight. These atoms are known as isotopes of that element.
A German chemist by the name of Otto Hahn conducted an experiment in 1938 in which he bombarded uranium with neutrons. The atomic number of the uranium he used was atomic number 92, but the elements he created were far lighter. They also had much lower atomic numbers. Astonished by the results of his experiment, he consulted with Lise Meitner a former colleague. She concluded that the neutrons had effectively split the nucleus of the uranium into two large pieces. They called the process of splitting the nucleus of an atom "fission." A chain reaction can occur if the splitting of the atom creates two new neutrons that then go on to split more nuclei, which in turn go on to split other nuclei.
Fission reactions are used both in atomic bombs and in nuclear reactors. They take place only with elements that are very heavy and only in certain isotopes. Fission comes about when the large nucleus of a heavy atom overcomes the nuclear forces that hold it together. In a fission reaction, extremely large amounts of energy are released. This is because the combined masses of the nuclei that are produced are less than the mass of the nucleus that is being split.
While there are a few different isotopes that will undergo fission if they are hit with high energy neutrons, uranium 235 (U-235) is the isotope that is more frequently used. The reason for this is that low-energy neutrons can be used to split U-235 apart into fragments. The products of nuclear fission are neutrons and the fragments, which are known as "daughter material." It is this daughter material that will decay because it has an excess of neutrons. This material is radioactive and is what is called nuclear waste.