It is possible to split an atom. When people refer to splitting an atom, it’s the nucleus of the atom that is being split. One process that splits the nucleus of an atom is called fission, which can happen spontaneously in heavier elements. Spontaneous fission basically involves a nucleus emitting a particle containing one or more protons or neutrons. One of the most commonly emitted particles is called an alpha particle, which consists of two neutrons and two protons. Whenever the number of protons in a nucleus changes, it becomes a different element.

Nuclei can also be split intentionally in laboratories. The nucleus is held together very tightly, so it usually takes a high-energy particle colliding with an atom to break it apart. Typically a high-energy neutron is used to initiate the process of splitting a nucleus. This process results in an overall release of energy so that once one nucleus is split, its products can cause the reaction to happen again. This is called a chain reaction, and it can be used to produce energy in a nuclear reactor (if it happens somewhat slowly), or an explosion (if it happens quickly).