May’s Molecule of the Month

You’ve all heard of oxygen. It makes up 21% of the gas in the atmosphere. It’s an element that all life relies on. Plants make it through photosynthesis. What might surprise you, given oxygen is everywhere, is how reactive it can be when it’s in the right form. The element oxygen can be turned into a dangerous form – ozone. But ozone also forms a layer around our planet that protects us! What makes ozone so special and what can it do?

Let’s start with oxygen.

Oxygen – perhaps the one element everyone is familiar with. The element some might say is the most essential for life! After all every breath allows our body to receive vital oxygen to keep us going. Even animals that live underwear obtain vital oxygen through their gills.

In fact, the levels of oxygen in the atmosphere have changed over the millions and billions of years since the Earth was formed. These changing levels of oxygen have had a massive impact on the evolution of life. At present, the atmosphere contains around 20% oxygen. However let’s go back around 300 million years ago, to a period known as the carboniferous. Oxygen levels in the atmosphere may have reached up to 35%! This amount of oxygen helped, directly or in-directly, to produce super large insects during this period.

Elements, atoms, molecules.

So far, the oxygen we have mentioned so far is specifically diatomic oxygen, or O2. This is the form of oxygen most people will recognise: two oxygen atoms bonded together. However, different forms of oxygen molecules can exist. These are known as allotropes[1]. It is possible for 3 atoms of oxygen to bond together to produce an O3 molecule. You may think that you have never heard or seen anything about O3. However, this molecule plays an important role in keeping the Earth safe from potentially deadly rays from space. In recent decades the loss of ozone has received attention and is the subject of scientific, and political debate.

This is because O3 is better known as ozone.

What is ozone?

Ozone is most famously known for the layer it forms in our atmosphere. This ozone layer protects us from a specific type of harmful ultraviolet (UV) radiation from the sun. However, ozone can be found throughout the atmosphere including the low-lying air we interact with. Ozone also has important uses in scientific research. Its reactivity can be extremely useful in helping to make or change a wide range of molecules.

Notice that we’re writing ozone as one word here as well. It is a common mistake to write O zone or O-zone. If you search for O-zone you’ll find a europop band from the 00’s who were famous for that one song that you’ll know of as ‘Numa Numa’.

However, before we get into all that, let’s have a look at what exactly what ozone looks like.

Ozone – the chemical

Normally, the oxygen we meet in daily life and are familiar with exists as two oxygen atoms bonded together. Oxygen is happy to be paired up in this way and is quite a stable molecule. Ozone, however, consists of three oxygen atoms connected together to produce a molecule with a bent shape very similar to water.

oxygen chemical structure
Molecular Oxygen
Ozone structure
Ozone – not quite single bonds and not quite double bonds!

What does ozone do?

At normal temperatures, ozone exists as pale blue gas. When dissolved in a liquid, or condensed into its liquid form, ozone forms a dark blue solution. But beware – liquid ozone can be very dangerous and can explode! It has a smell similar to chlorine, a sort of bleach like smell. The smell is so strong that some people can detect one molecule of ozone amongst 10 million other molecules. That is one very strongly smelling molecule!

It’s not just the smell of ozone that is particularly strong; this molecule is very reactive and can be described as a strong oxidizing agent. What does this mean? In simple terms, ozone is able to easily accept the electrons from other molecules. This turn reduces the other molecule and helps to make chemical reactions occur. This ability makes ozone reactive to all kinds of molecules, including metals, nitrogen and sulphur. It will even react with pure carbon, an element that is normally very difficult to get to react.

Ozone’s reactivity lends itself to the majority of its uses.  In general, these tend to fall into two categories: synthesis and disinfection. The ability of ozone to react with many, many things makes it the ideal tool in industry to help make all kinds of molecules, including drugs and lubricants. Here, the key feature of ozone is its ability to cut bonds between carbon atoms, a very useful tool when you want to build, re-model or simply just remove a small part of a molecule.

Can we use ozone for anything?

The strong reactivity of ozone also makes useful as a disinfectant or sanitizer. However, this is usually for large scale cleaning in large buildings. You are very unlikely to ever buy a small bottle of ozone to spray around house. Please don’t look and definitely don’t buy anything claiming to be ozone in a bottle! Talking of bottles, the purification of bottled water is one of the ways ozone is used to clean and sanitize. In fact, ozone is used for water purification, where it can replace the more traditional use of chlorine. The use of ozone instead of chlorine does also carry certain benefits.

It’s not just water either; ozone can be used to clean whole rooms or even buildings. Obviously when used in this way, the rooms are cleared and sealed off. This however, provides a rapid and effective way to clean hospital rooms after surgery.

The Ozone Layer

One of the most important roles that ozone plays occurs in the place where ozone is best known. Way up in the atmosphere, or more specifically, the ozone layer. Just so we are all clear, let’s very quickly outline what our atmosphere consists of. We can split our atmosphere into five main layers. The bottom layer, starting from the surface of the Earth is the troposphere.  This is followed by the stratosphere, mesosphere, thermosphere, and then finally ending with the exosphere. The majority of commercial airlines and weather occurs in the troposphere.

Where is the ozone layer?

The ozone layer is found in the stratosphere, roughly 15-35 km above the Earth’s surface. Despite its name, the amount of ozone in the ozone layer is rather small compared with the other gases there. In fact there is much, much more O2 in the ozone layer, than ozone itself. However, this is the part of the atmosphere where the concentration of ozone is greatest. So what? Why is the ozone layer so important and how does it protect us?

ozone layer protect from uv sunlight
The Ozone Layer

The ozone layer protects us

Let’s start by thinking about the sun. For those who don’t know, that great big shining thing in the sky during the day, is the sun. A medium sized star that produces immense about of heat and light by fusing hydrogen nuclei into helium. When most of us talk about light, we are actually referring to visible light, a specific part of the electromagnetic spectrum [2]. If we increase the wavelength of visible light we move into the infrared part of the spectrum.

Alternatively, if we decrease the wavelength of the light, and increase its energy, we move into the ultra-violet (or UV) part of the spectrum. As it is higher in energy, UV light can be very damaging to all things biological: animals and plants, including humans – more specifically our skin, cells and even DNA. This makes the UV light, produced by the sun, potentially very dangerous should it reach us on the surface of the Earth. This is where the ozone layer protects us.

The ozone layer protects us by absorbing UV light. We can be more detailed and split UV light into 3 types: UV-A, UV-B and UV-C. UV-C is filtered out by both O2 and ozone, whilst UV-B is mostly removed by ozone. However, a small amount of UV-B does reach the Earth’s surface, where it plays an important role in helping our skin to produce vitamin D. UV-A is not affected by ozone and reaches the Earth’s surface, although this type of UV is less harmful. But don’t be fooled. By “less” harmful, we mean compared with the other types of UV, because UV-A can still result in physical damage, especially during intense and long exposure.

If the ozone layer protects us, is ozone itself good or bad for us?

Ozone then appears to be good for our health. It helps remove the majority of high energy UV light that would otherwise burn and cause us serious problems. Its similarity to the more familiar O2, must therefore mean ozone is good for us, right? Does it keep it’s reputation (from the ozone layer), as something that protects us? After all, molecular oxygen (O2) to vital to our lives. Unfortunately, no.. Ozone is actually a pretty nasty molecule.

As a gas, inhaling ozone takes it directly into the lungs. Once there, it can irritate and cause the lungs to become inflamed. This can lead to shortness of breath, wheezing and coughing. Encounters a lot of ozone for a very short amount of time, every now and then, is known as acute exposure.

Chronic exposure, or long-term interaction with ozone, also comes with severe health effects. It can increase the risk of getting, or dying from, respiratory illness such as asthma. If you want to know more about asthma and its treatment, here’s last month’s molecule of the month, salbutamol! For those with such pre-existing conditions, constant exposure to ozone is even more dangerous.

But if ozone is up in the sky, we’re fine right?

But hang on, if ozone is normally found in its layer high up in the atmosphere, doing the whole ‘protects people from UV thing’, then what is it doing here on the surface? Well, low lying ozone has in recent decades become a problem, as is classed as an air pollutant. Waste produced from burning fossil burns interacts with UV rays during the day to produce ozone in the air around us. Ozone is important to protect us, but it’s best left high up in the atmosphere. We don’t want it at lower altitudes where people are likely to breathe it in.

How is ozone made?

We’ve seen the important role ozone plays in our atmosphere, discovered the many other uses it has and realised the health problems ozone can cause. We haven’t though really spoken about how ozone can be made. After all, if oxygen prefers, and is usually found as O2, then how is O3 formed? Well, to see this, we have to return to the lofty heights of the stratosphere, the ozone layer and add some UV light into the mix.

The O Zone and ozone are different things!

It is probably no surprise that many chemical reactions can be started, or made to go quicker, with heat. Heat provides the molecules with enough energy to react. In general, the more heat your molecules have the more chance they will react.

However, chemical reactions can also be started with light. Light provides the energy needed for a reaction to occur (from a simple point of view anyway). Up in our atmosphere, energetic UV rays can break up molecules of O2 to form atomic oxygen, simply written as O. [3] These atomic oxygens are very reactive, and can easily collide into another molecule of O2 to create O3, a molecule of ozone.

Does ozone break apart?

This is not the end of the story though. Those same UV rays, can also break up molecules of ozone back into O2 and O. This time, however, if the reactive oxygen atom collides with a molecule of O3, it produces two molecules of O2. Furthermore, it is possible for two atoms of oxygen to bump into each other and form a molecule of O2.

The main point here, is that the production of ozone from O2 and O, and the destruction of O3 back to O2 and O is occurring all the time. Normally, there is a nice balance to these reactions that means the amount of ozone being made is roughly the same as the amount being destroyed, and so the ozone layer remains intact, with this constant soup of oxygen related collisions happening. Normally…

You see the destruction of ozone can be made to go faster by the presence of other reactive (radical) species such as hydroxide (OH), nitric oxide (NO), chlorine (Cl) and bromine (Br). Again, normally this is all natural and does not upset the delicate balance between construction and destruction of ozone. But… an increase in the number of these reactive species, would increase the destruction of ozone and therefore thin the layer. This is exactly what has occurred through human activity, more specifically from the use of chlorofluorocarbons – CFCs.

The ozone layer protects us, we should protect it.

For many, many years CFCs were commonly used in fridges, aerosols and other applications. Intense use of CFCs has caused a depletion in ozone, causing it to get thinner. CFCs created holes in the ozone layer, allowing harmful UV rays through that would have normally been removed. This, along with global warming and climate change, is one of the major global and environmental challenges we face. It also emphasises the importance of ozone – this small three atom molecule, floating above us all. The ozone layer protects us and will do so as long as we protect it, too.

So many molecules essential to the Universe, to the Earth and to life are actually very small and very simple. H2O, O2, CH4, H2, CO2, are just some of the examples of such molecules. Ozone is no exception. Despite not being as familiar or abundant as it’s two atom cousin, ozone is vital for protecting biological species on Earth from the ravages of ultra-violet light. So next time you hear about ozone, the ozone layer or even just oxygen, you will hopefully be a bit more familiar with it all.

[1] Allotropes are different forms of the same element that exist in the same state of matter. You will be familiar with the allotropes of carbon, which includes graphite, diamond and carbon nanotubes, all of which are solid. Likewise, O2 and O3 are both allotropes of oxygen, and are both gases. BACK TO POST

[2] The electromagnetic spectrum refers to a range of waves of different wavelength and energies (long wavelength corresponds to lower energy). This includes (from longest to shortest wavelength): radio waves, microwaves, infra-red, visible light, ultraviolet, X-rays and gamma rays. BACK TO POST

[3] In simple terms, a radical is an atom or molecule that has an unpaired electron, essentially a single lone electron. This makes radicals extremely reactive to other radicals or even other non-radical molecules. BACK TO POST

Graphics by

Phil Jemmett

Phil Jemmett

Phil has a Ph.D. in Biophysical Chemistry at the University of Birmingham and now works for WMG at the University of Warwick – tinkering and building things for outreach projects! He makes the graphics and plots experiments for you all to try out at home.


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