The chapman cycle is a constant cycle between oxygen molecules and their interaction with ultraviolet rays, which forms ozone. It is known as a cycle because it is the constant repetition between two different allotropes of oxygen. The ozone is created when ultraviolet light react with oxygen molecules which creates ozone.
When oxygen molecules in the atmosphere are exposed to UV light which has a wavelength of 120 - 240nm. The high energy light breaks the bonds within the oxygen molecule forming two separate oxygen atoms: known as free radicals. O2 + UV light (~240nm)--> O + O
The two oxygen atoms are energized and collide with oxygen molecules, which bond and form O3 (Ozone). When an ozone molecule is exposed to UV light, an exothermic reaction occurs (negative enthalpy), forming 1 free radical and an oxygen molecule. The free oxygen atom will then collide with an ozone molecule breaking more bonds and ultimately have two oxygen molecules as there was at the beginning of the equation.