# What property allows ozone to absorb uv light

Why can ozone act as a barrier of UV light, so that we don't get irradiated by the sun's energy, and not any old molecule? In a recent exam I sat, one of the questions was "Name one physical property of ozone and contrast it with oxygen" and under exam pressure I spurted this onto the page: "Because the coordinate covalent bond in ozone is weaker than a traditional covalent bond, the bond is free to 'wiggle' about a bit. This means that the ozone can absorb a range of frequencies of photons, as a completely rigid molecule like oxygen would only have very specific frequencies which it could absorb." How much of this 'guess' is grounded in fact, or in other words, what is the actual mechanism that allows ozone to protect us from UV rays and not any old molecule?

• Ozone is less stable than oxygen. When UV radiation reaches ozone layer, the ozone molecules decompose to form oxygen molecules which are more stable. So even before UV radiation can reach earth's surface, its energy is used up in ozone layer for forming more stable oxygen molecules. – Aditya Dev Sep 1 '15 at 12:13
• Now you may think that due to UV radiation ozone layer depletes quickly. But in reality, reverse reaction also takes place. A free oxygen atom combines with a normal oxygen molecule to form ozone. The free oxygen atom comes from decomposition of ozone. – Aditya Dev Sep 1 '15 at 12:19
• @Aditya Dev With links and some extra stuff you could make answer with it. – Mithoron Sep 1 '15 at 13:57

Shortwave UV light (100-240nm) has enough energy to knock an electron out of an $\ce{O_2}$ bond creating an excited molecule $\ce{O^{excited}_2}$ which decomposes to two free radicals of oxygen. Each of the free radicals then combines with different oxygen molecules to create a net of two ozone molecules.

The crux here is that the electron is not "knocked out" of the oxygen molecule, the electron is just moved to a higher energy orbital of the same oxygen molecule. Since orbitals are quantized it takes a very specific energy to make the transition happen.

Now there is a second reaction. Longwave UV light (240nm - 350nm) has enough energy to knock an electron out of an $\ce{O_3}$ bond creating an excited molecule $\ce{O^{excited}_3}$ which decomposes to a free radical of oxygen and an oxygen molecule.

Thus oxygen protects the ground from shortwave UV, and ozone from longwave UV. Neither alone would protects us from all UV as effectively.

Now the rest of the story is that all molecules bouncing around in the air are not in the same energy state due to atom-atom vibrations. So the UV absorption occurs as a "band" rather than as a "fine line" like in the absorption spectra of an atom in free space.

Firstly, ozone is an unstable compound and decomposes at about $573K$ to form oxygen. When UV radiation reaches ozone layer, ozone molecules present at the layer absorb its energy and decompose to form oxygen which is more stable than ozone. One ozone molecule decomposes to form one oxygen atom and a normal oxygen molecule. Hence the energy of UV radiation is absorbed in ozone layer even before reaching earth's surface. That's how ozone layer protects us from UV radiation.

Now, this does not deplete ozone layer since reverse reaction can also take place. That is, high energy UV radiation hits oxygen molecules and each decompose into two free oxygen atoms. Each oxygen atom combines with one oxygen molecule each and forms ozone molecules.

You should also note that the bond order of ozone is $1.5$ and that for oxygen is $2$.

You may also want to read about depletion of ozone layer. Here is a link: Ozone hole

This is not some intrinsic property of ozone. In fact, any molecule would absorb a range of frequencies of photons. It just so happens that O3 covers the very range that is not covered by other components of our atmosphere, and thus saves us from the detrimental effects of UV rays of that range.

The reason ozone "filters" UV light at certain wavelengths is due to creation and destruction of ozone via UV light. This is the reason we have an ozone layer in the stratosphere.

UV light creates ozone at wavelengths less than ~240 nm. While UV light from ~240 - 315 destroys ozone. In the process of creating and destroying ozone the UV light is absorbed in those wavelengths.