High above the Earth, in the stratosphere, a constant chemical drama unfolds. This process begins with diatomic oxygen ($O_2$), the stable, familiar form of oxygen we breathe. In the thin air of the stratosphere, these molecules are bombarded by high-energy ultraviolet (UV) radiation from the sun.

Specifically, it's the most energetic portion of the UV spectrum, known as UVC, with wavelengths between 100 and 280 nanometres (nm), that drives the reaction. This isn't just any light. This radiation carries enough energy to act like a pair of molecular scissors, snipping the strong double bond that holds the two oxygen atoms together. This process is called photodissociation or photolysis.

The energy required to break this bond, known as the bond dissociation enthalpy, is about 498 kJ/mol. Wavelengths of light shorter than 242 nm carry enough energy per photon to overcome this barrier. The result is two individual oxygen atoms, each with an unpaired electron. These aren't stable atoms anymore; they're oxygen radicals – highly reactive and eager to bond with other molecules.

This whole process is critically dependent on altitude. Why does it happen primarily in the stratosphere and not closer to the ground?

Higher up, in the mesosphere and beyond, there is plenty of intense UVC radiation, but the atmospheric pressure is extremely low. There are simply not enough $O_2$ molecules to absorb the incoming energy effectively.

Lower down, in the troposphere where we live, the air is dense with $O_2$. However, almost all the high-energy UVC radiation has already been absorbed by the oxygen and ozone in the stratosphere above. The stratosphere is the sweet spot where the concentration of oxygen is still significant and the flux of incoming UVC radiation is strong enough to drive photolysis on a massive scale.

This initial splitting of $O_2$ is the critical first step. It produces the raw ingredient—atomic oxygen—needed to build ozone ($O_3$). This entire sequence of reactions is known as the Chapman Cycle and is fundamental to understanding the structure and stability of the ozone layer.

This crucial first reaction sets the stage for the formation of the ozone layer, which protects life on Earth by absorbing the vast majority of harmful UV radiation.