Is Oxygen O2 or O: The Molecular Structure of This Element

Oxygen is everywhere, and we cannot imagine life or survive without it. Yet, as omnipresent as it is on Earth, what fundamentally constitutes oxygen can leave the best of us gasping for air. Namely, is oxygen O₂ or just O?

Oxygen exists as two atoms in its stable molecular form and is written as O₂ or O2 to distinguish it from an atom of oxygen O. Technically, both O₂ and O are forms of oxygen. O is the chemical element of oxygen, while two oxygen atoms chemically bound together produce O₂, a colorless, tasteless, and odorless gas.

The combination of O and O₂ produces ozone, O₃. Oxygen, in the form of O₂ and O₃, only appeared halfway through the planet’s history, causing an explosion in biodiversity. 

While it is hard to comprehend a world without oxygen, for approximately the first half of the four-and-a-half-billion-year history, Earth had no O₂ as part of its atmosphere.

If you are just as puzzled by the phenomenon of oxygen, make sure to read this article. You will discover what oxygen consists of and why it is one of Earth’s many marvels.  

Is Oxygen O₂ or Just O?

O₂ is the proper way to notate the compound that humans breathe. It is also known as oxygen gas, dioxygen, and diatomic oxygen (source). 

The molecular formula for oxygen is O₂. This formula shows the exact number and type of atoms in a molecule. Still, it does not reveal any information about the arrangement of atoms. 

The structural formula of oxygen is O=O. A structural formula is used to indicate the number of atoms and their arrangement in space (source).

The most common form of oxygen found is O2 since neutral oxygen (O), by default, seeks two more electrons to complete its outer valence shell. 

What is Oxygen? Understanding The Chemistry

Oxygen is a chemical element that contains only one type of atom. Its atomic symbol is O, and its atomic number is eight. Oxygen is a colorless, tasteless, and odorless gas (source).

The credit for discovering oxygen is shared by three chemists: the Englishman Joseph Priestley (1774), the Swede Wilhelm Scheele (1773), and the Frenchman Antoine Lavoisier (1777) (source).

Oxygen is vital for life as most living things — humans, animals, plants — breathe in oxygen for survival. While breathing out, carbon dioxide is released and absorbed in the atmosphere. 

In a separate process, photosynthesis, plants take in carbon dioxide and release excess oxygen in the atmosphere (source).

Oxygen is very reactive and quickly binds to other elements. For instance, hydrogen (H), carbon (C), and iron (Fe) form water (H₂O), carbon dioxide (CO₂), and the oxygen-containing mineral goethite, a component of rust (source).

While oxygen is non-flammable, it is an oxidizer, which means it will feed a fire. The fire triangle includes heat, a fuel substance such as wood or charcoal, and oxygen. The result is combustion, a chemical process where a substance interacts with oxygen to produce heat (source).

Oxygen comprises 21 percent of the atmosphere by volume. This is halfway between 17 percent, below which breathing for unacclimated people becomes difficult, and 25 percent, above which many organic compounds are highly flammable. 

The element and its compounds make up roughly 49 percent by mass of the Earth’s crust, and about two-thirds of the human body (source).

What Is Not Oxygen?

Oxygen is not the same as the air around us. The air you breathe in contains a mixture of different gases, including oxygen. 

The Earth’s atmosphere contains about 21 percent oxygen. The rest is made up of approximately 78 percent nitrogen. Air also has small amounts of other gases, such as carbon dioxide, neon, and hydrogen (source).

Before we unpack oxygen in even greater detail, let’s clarify what a molecular structure is and why it is important. 

What Is Molecular Structure?

Molecular structure is an important concept in chemistry, yet not an easy one to explain. Let’s start with its chemical definition and then bring in a helpful resource, the Oxford English Dictionary, to look at some basic chemistry terminology. 

Molecular structure, or molecular geometry, is the three-dimensional structure or arrangement of atoms in a molecule. It specifically looks at the atoms’ placement in a molecule, not the electron-pairs (source).

The Oxford English Dictionary defines an atom as the smallest particle of a chemical element that exists (source). It also defines a molecule as two or more atoms bound together that retain their chemical properties (source).

Even smaller than an atom is an electron. An electron is a subatomic particle, the lightest stable substance that we know of, with a negative electric charge (source). A proton is a charged particle, too, only with a positive electric charge. 

The Molecular Structure of Oxygen

One neutral oxygen atom (O) consists of eight protons and eight electrons. 

The electrons in an atom are arranged in shells that surround the nucleus, with each successive shell being farther from the nucleus. The area closest to the center of the atom can contain up to two electrons. There is space for 8 electrons in the subsequent grouping of electrons, also known as valence shell (source). 

The electronic configuration of oxygen is 1s²2s²2p⁴ and indicates where the electrons are located around the atom’s nucleus — two core electrons that are non-reactive and six valence electrons (source).

The Valence Shell Electron-Pair Repulsion (VSEPR) Theory 

This theory can predict the structure of nearly any molecule. This theory assumes that electron pairs in the outermost shell of a central atom tend to repel each other and therefore keep maximum distance to minimize this repulsion (source).

Oxygen has six valence electrons. This is interesting since atoms tend to be more stable when they have a filled outer shell. Since there is room for eight electrons, each oxygen atom is actively seeking to get more electrons to complete its valence shell (source).  

The result is that two oxygen atoms will each share two electrons to form two covalent bonds and make an oxygen molecule; in other words, O₂ (source).

What is O₃?

An atom of oxygen (O), together with oxygen gas (O2), produces three atoms of oxygen, O₃, also known as ozone. Ozone appears as a colorless to bluish gas with a characteristic odor. Ozone can be both good and bad, depending on where it is found on Earth.

Stratosphere ozone, also labeled “good ozone,” forms an essential barrier against solar radiation and is naturally found in the upper atmosphere (source).

Ozone in the stratosphere has been partially damaged by human-produced chemicals. Rather than referring to it as “ozone holes” or the “hole in the ozone,” think of the damage in terms of tiny cracks in the ozone. The ozone layer is thinnest near the poles (source).

Bad ozone occurs at the Earth’s lower atmosphere, the troposphere, near ground level. It is associated with smog and occurs when air pollutants, such as emissions from cars and chemical plants, react chemically with sunlight. Ozone at ground level, therefore, is a harmful air pollutant (source).

The effect of climate change with subsequent changes in weather patterns has also left its mark on the formation and rise of bad ozone with a detrimental impact on human health (source). 

Oxygen: One of Earth’s Many Marvels

Earth without oxygen? This seems hard to fathom. Especially since Earth is the only planet that we know of that contains oxygen.

Interestingly enough, the Earth barely contained any trace of oxygen in its atmosphere during the first 2.2 billion years of its life. What did life look like before O₂?

Life Before Oxygen

Life before oxygen gas bears no resemblance to life as we know it. As there was no oxygen gas in the atmosphere, no living, breathing creatures, such as insects, fish, or humans, were present. It was inhabited, though, but only by single-celled organisms. 

For decades, scientists assumed early Earth was built on the basis of a deadly atmosphere cocktail, filled with noxious methane, carbon monoxide, hydrogen sulfide, and ammonia.

However, more recent studies indicate the existence of an atmosphere that included water, carbon dioxide, and sulfur dioxide (source). 

And while O₂ had not yet been accumulated in the atmosphere, oxygen was always present in compounds in Earth’­s interior, atmosphere, and oceans. For things to change, more oxygen was required, more than what was used up. Only life itself was capable of this (source).

The Debut of Oxygen 

The debut of oxygen occurred around two and a half billion years ago. It accumulated from the photosynthesis of blue-green algae, also known as cyanobacteria. 

Photosynthesis is the process used by plants, algae, and bacteria, to convert sunlight into chemical energy. The carbon dioxide produced by these living organisms gets released as oxygen into the atmosphere (source).

At one point, around two to four billion years ago, the Earth’s atmosphere accumulated enough oxygen to create a permanently oxygenated atmosphere, a transition widely known as the Great Oxidation Event (source).

That the rise of oxygen on the planet took several millions of years still puzzles scientists. To add to the confusion, scientists discovered that cyanobacteria existed long before the Great Oxidation Event. What caused the “sudden” rise in oxygen? What changed?

A study led by the University of Washington attributes the rise in oxygen to the reaction of volcanic gases with oxygen. 

For several million years, oxidizable volcanic gases canceled out the oxygen produced as a result of photosynthesis. Once fewer volcanic gases were released, as a result of the oxidation of the Earth’s mantle, free oxygen flooded the air (source).

The effect was profound. Once O₂ appeared in the atmosphere, although at extremely low levels for the next two billion years, it fundamentally changed the trajectory and course of the Earth and the evolution of new forms of life on it. 

The Mystery Continues: Why so much Oxygen Now? 

How the planet managed to contain an abundance of roughly 21 percent of oxygen in the atmosphere remains even a greater mystery. 

What is clear is that it has been a tumultuous ride. For 2 billion years, the levels of oxygen in the Earth’s atmosphere varied, rising as high as 25 percent and dropping to 10 percent in that 2-billion-year period. 

Major mass extinctions correspond with oxygen levels dropping below 10 percent. Scientists have primarily contributed these changing oxygen cycles to the supercontinent cycles of drifting and colliding continents (source).

Once oxygen levels stabilized in the atmosphere, it pivoted the planet’s development, giving rise to many new forms of life. It set the course for the Earth’s rich biodiversity and created space for living, breathing creatures to inhabit the planet.

While the role of oxygen in the planet’s evolution is unmistakable, a study by UC Berkeley geochemists indicates that high levels of oxygen, comparable to the 21 percent today, were not critical to the origin of animals (source). 

Preserving the Earth’s biodiversity is largely achieved through environmental stability and requires quick action against climate change (source).

If history has shown us anything, who knows whether the Earth will hold on to its remarkable supply of oxygen, or run low again?

Final Thoughts

Oxygen is normally found as a molecule, O₂, which we sometimes call dioxygen. It is different from an atom of oxygen (O) and ozone (O₃), a molecule of three oxygen atoms. Despite all three being oxygen, they contain different properties. 

Oxygen is essential to life, beginning with the air we breathe, its presence in our bodies, and its offering in global sunscreen through the ozone. Yet, its sheer presence remains a mystery. 

While early life was devoid of almost any trace of oxygen in its atmosphere, the photosynthesis of cyanobacteria changed the course and trajectory of the planet. It created Earth’s biodiversity and led to the most remarkable and unique living planet in the universe.