In chemistry, fission is the process by which a molecule splits into two constituent parts. This occurs when one of the chemical bonds between atoms in the molecule is broken.

In homolytic fission, the two electrons from the broken bond are shared between the resulting species. This means that each species contains an unpaired electron in an outer shell. They are therefore highly reactive, and are known as free radicals. Homolytic fission occurs when the two atoms being separated have a similar or identical electronegativity - that is, they have roughly the same ability to attract electrons to themselves.

One example is the splitting of halogen molecules (X2) by ultraviolet radiation. Chlorine, Cl2, splits into two chlorine atoms, 2Cl•. These atoms are free radicals and their reactivity may be used to initiate useful chain reactions.

In heterolytic fission, the two electrons from the broken bond go to the same species. This occurs when one species is significantly more electronegative than the other. Heterolytic fission results in a negatively charged anion, which received both electrons, and a positively charged cation, which received neither.

For example, water, H2O, may split heterolytically into a hydroxyl ion, OH-, and a hydrogen ion (a proton), H+.