Oxygen is the 8th element on the periodic table, and is the 3rd most abundant element in the universe. Although the amount of oxygen in the universe is dwarfed by hydrogen and helium, oxygen is the most common element on Earth, and it is a crucial component of life. Plants release oxygen in the process of photosynthesis, and humans and other animals intake the oxygen when they breathe. Oxygen makes up 46% of the Earth's crust, with silicon making up 27%, so unsurprisingly the most abundant minerals present on Earth are a combination of the two, known as silicates.
Some properties of oxygen include:
|Density (at 0oC)||1.429 g/L|
|Boiling point||90.2 K|
|Melting point||54.8 K|
Oxygen is by far the most common element on Earth, making up around 21% of the atmosphere, 90% of the Ocean and two thirds of the human body. However free oxygen wasn't always present in the atmosphere, and around 2.5 billion years ago the only life on Earth were very small bacteria that didn't require oxygen to live. These organisms were in fact poisoned by oxygen, so when a photosynthetic life form called cyanobacteria began to flourish by converting sunlight into energy and producing oxygen, the other life-forms began to die off. This is known as the Great Oxygenation Event, and made way for the life-forms of today. For more information visit our page on atmospheric oxygen.
Oxygen is found in many forms on Earth, most commonly bonded with hydrogen to form H2O, water, but is also a key part of other molecules important to life. Oxygen combines with almost every other element to form "oxides": For example it combines with silicon in the Earth's crust to form silicon dioxide found in granite and quartz, with iron to form iron oxide found in rust, and with calcium and carbon to form calcium carbonate found in limestone. Visit UC Davis' ChemWiki to learn more about how oxygen interacts with the other elements.
Aside from oxygen's role in the chemistry of life, a different form of oxygen is also vitally important on our planet. Ozone (O3) is found in the upper layers of the atmosphere, forming a thin layer around the Earth, known as the Ozone layer. It is important because it absorbs most of the Sun's ultraviolet radiation that is harmful to life. Ozone is created when molecular oxygen (O2) is split apart by sunlight into single oxygen atoms, which then recombine to form ozone. Ozone is destroyed when it reacts with certain molecules, some of which are put into the atmosphere by humans. The destruction of ozone caused by humans is contributing to the known "ozone hole" which is a big concern.
Oxygen is important for its role in the burning of fuels, which is a large focus on this encyclopedia. Fossil fuels such as methane go through the process of combustion in which they react with oxygen, which produces carbon dioxide, water vapour and heat as shown in Figure 3. This process supplies the world with most of its primary energy - around 95%. This process is actually quite fascinating; dead plants and animals are what make the fossil fuels, live plants are what make the oxygen, and the result is energy for human's needs.
The video below is from the University of Nottingham's periodic videos project. They have created a complete suite of short videos on every element on the periodic table of elements. This video talks about oxygen, but also discusses ozone.