How do high-energy radiation and ultraviolet light cause chemical bonds to break?

High-energy radiation and ultraviolet (UV) light break chemical bonds by altering the electronic structure of molecules, often leading to the formation of highly reactive radicals.

Photochemical Processes

When molecules absorb UV light, they enter an excited state where the distribution of electrons—which provide the bonding forces holding atoms together—changes drastically. In a process known as photodissociation or photolysis, this absorption provides sufficient energy to disrupt specific chemical bonds, causing the molecule to split into two fragments. These fragments are typically radicals, as each retains enough electrons to form half of a bond, making them chemically unstable and highly reactive.123

High-Energy Radiation

High-energy radiation causes bond breakage primarily through ionization, where energy input ejects an electron from a molecule, creating a positive molecular ion. This process can be direct or indirect:45
* Directly ionizing radiation (e.g., protons or alpha particles) disrupts molecular structures as it passes through matter.5
* Indirectly ionizing radiation (e.g., X-rays or gamma rays) transfers energy to produce fast-moving particles that subsequently damage molecular structures.5

These interactions often result in the transient formation of free radicals, which can lead to further chemical changes or the breakage of complex structures like DNA.35