When two atoms in a molecule having a chemical bond (single covalent) with 2 electrons in the bond, are broken we have a fission reaction. It may be triggered by different conditions and may be catalysts.
When two atoms with a bond A - B break and form radicals A and B, each with one electron on it, the reaction is called Homolytic fission. This fission may be triggered by UV light or some other agent. Here A and B are not electrically charged.
When the two atoms separate with A- taking two electrons and B+ not taking any, then it is heteolytic fission. Here the ions are electrically charged. They are both different elements/species and have different behavior.
It can happen with Hydrogen H2, Cl2, HCl, O2 etc.