Ozone

Oxygen comes in many guises. You have the singular atom/radical version, which is very keen to react so doesn't last long in the atmosphere. Then you have the O₂ molecule, which is by far the most ubiquitous and makes up most of the 21% present in the atmosphere. And then you have ozone, chemical formula O₃, which is named only for its smell. And there are others, but this blogpost is on ozone, so let's move on.

In industry, ozone is used in decontamination such as through improving the shelf-life of food products, as well as in treating water and sewage. There had been research into the effectiveness of ozone in treating Covid-19 patients - here's a paper on it - and it's so good at decontamination precisely because it's so unstable, which I'll explain in a few paragraphs. It decomposes when in contact with the air and water, which enables it to break up microorganisms so effectively. But most people know ozone for the eponymous layer.

Ozone is especially good at blocking UV radiation, especially UV-B. This is through a sacrificial process, where the ozone is broken up by the UV and this reaction takes place:

O₃ → O₂ + O 

Eventually, the oxygen molecule and atom will react together and reform the ozone molecule, and it's this continual process that helps sustain the crucial ozone layer.

Without the ozone layer, we would all be dead. That's slightly ironic, considering it's an irritant that can damage crops and is a component of smog. However, without it, the Sun's radiation would be far more potent, thus overcooking us and giving us cancer. 

There is ozone on the ground, but not as much as there is in the stratosphere, with its peak concentration at 15 parts per million, and it is especially prevalent at the poles. This peak comes at an altitude of 32km, and beyond this, it slowly begins to decline towards its ground level concentration.

Ozone's consists of one double bond and a single bond between the oxygen atoms respectively. This causes the central atom to have one lone pair of electrons, and the adjacent atoms have two and three lone pairs respectively. This difference in charge results in the formation of a dipole, which is why ozone is very reactive and easy to break up. Halogens in particular can attack the ozone molecule and cause the formation of the more stable O₂ molecule, and other pollutants like nitrous oxides can do the same. Take this reaction between a chlorine radical and an ozone molecule, which is made up of two propagation steps:

Cl● + O₃ → ●ClO + O₂, and

●ClO + O₃ → Cl● + 2O₂.

Obviously ozone breaks down due to UV radiation, and this reaction merely catalyses the whole process. The main problem is when this reaction occurs at a far greater rate than it should, as more ozone is lost than is replenished - enter Thomas Midgeley, whose solution to a problem ended up being far more harmful than expected.

Midgeley has been described as a "one-man environmental disaster" for his work with leaded fuel, and his work with chlorofluorocarbons (CFCs) shares a similar status. They came about initially as a refrigerant as unlike other agents like ammonia, they weren't toxic. As a result, they became a sensation in the 20th century, also being used in fire extinguishers, bug sprays and air conditioning. They would have been fantastic if it wasn't for the fact that chlorine and fluorine are quite reactive elements, so if they got into the environment, they could easily react with the ozone layer. To be fair to Midgeley, he may not have realised they were this harmful, and it was only until the 1960s when research was conducted into the effects of CFCs. But they were certainly harmful, and in 1987, many countries came together to sign an agreement to stop the production of CFCs. 

At this point, there was already a large ozone hole over the Antarctic, though since then, the hole has gradually started to patch up, thanks to the lack of CFCs. Just one example of how chemicals that seem great can also be harmful; though maybe one could argue it was the ubiquity of CFCs that meant there was greater damage to the ozone layer. Either way, CFCs will not be missed. 

Ozone, however, would most certainly be missed if it suddenly disappeared. From making water safer to use and keeping us alive, it's quite good. Not bad for an unstable allotrope of oxygen.