Allotropes of oxygen

Oxygen can exist in several different forms, called allotropes. The most common one is molecular oxygen (O₂), which makes up about 21% of the air we breathe. This form is also known as dioxygen or triplet oxygen. Another important allotrope is ozone (O₃), which is very reactive and helps protect the Earth from harmful sunlight.

There are also less common forms of oxygen. Atomic oxygen (O₁) is a free radical, meaning it has an unpaired electron and is very reactive. Singlet oxygen (O₂^*) is a special, long-lasting form of molecular oxygen. Tetraoxygen (O₄) is another unusual, long-lasting form of oxygen.

Solid oxygen can exist in six different colored phases. One of these is called octaoxygen (O₈), which is red, and another phase behaves like a metal. These different forms show just how versatile and important oxygen is in our world.

Atomic oxygen

Atomic oxygen, written as O or O₁, is very reactive because the tiny oxygen atoms like to join with other molecules nearby. On Earth, this type of oxygen only exists for a very short time. But in outer space, where there is strong ultraviolet radiation, a lot of oxygen can be found in this atomic form, especially in low orbits around Earth.

Scientists have found atomic oxygen on Mars using tools like Mariner, Viking, and the SOFIA observatory.

Dioxygen

Further information: Oxygen

Oxygen is most commonly found in the air as a gas made of two oxygen atoms, called O₂. This form is very important for life because animals and many other living things need it to get energy from food.

There is another special form of oxygen where the atoms have different energy. This form can glow and is used in some kinds of lighting. It can be made using special lights or certain chemical reactions.

Ozone

Main article: Ozone

Ozone is a type of oxygen made of three atoms, written as O₃. It is a pale blue gas that is very reactive. Ozone can hurt materials like rubber and fabric and can damage lung tissue. You might smell a sharp, chlorine-like odor near electric motors, laser printers, or photocopiers because ozone is created when air is exposed to electrical sparks.

Ozone helps protect the Earth by blocking harmful sunlight in the upper atmosphere. However, when ozone forms near the ground, it can be unhealthy for some people, especially children and those with breathing problems. Ozone is unstable and usually changes back to normal oxygen. There is also a theoretical form of ozone where the three oxygen atoms form a triangle, but this has not been observed in nature.

Tetraoxygen

Main article: Tetraoxygen

Tetraoxygen is another form of oxygen that people thought might exist since the early 1900s. It was finally found in 2001 by scientists at the University of Rome. This molecule, written as O₄, seems to be made of two pairs of oxygen molecules that are held together loosely.

Pentaoxygen

Scientists have studied a form of oxygen made of five atoms. They think it might look like a ring, similar to a molecule called cyclopentane.

Hexaoxygen

Scientists think that a form of oxygen made from six atoms, called hexaoxygen, might look like a ring shape similar to cyclohexane. It could exist in different forms, like a chair shape or a twist shape.

Phases of solid oxygen

Main articles: Solid oxygen and Octaoxygen

There are six different forms of solid oxygen. One of these forms is a dark-red group of eight oxygen atoms, called O₈. When oxygen is pressed very hard, about 96 GPa, it changes and becomes like a metal, similar to how hydrogen does. At very cold temperatures, this form can also act like a superconductor.