MOLECULAR OXYGEN: STRUCTURE, PROPERTIES, USES - CHEMISTRY - 2025

The molecular oxygen or dioxygen, also called diatomic oxygen or gas, is the most common elementary way is this element on Earth. Its formula is O ₂, being therefore a diatomic and homonuclear molecule, totally apolar.

The air we breathe is made up of about 21% oxygen as O ₂ molecules. As we ascend, the concentrations of oxygen gas decrease, and the presence of ozone, O _{3, increases}. Our body takes advantage of O ₂ to oxygenate its tissues and carry out cellular respiration.

Without oxygen enriching our atmosphere, life would be an unsustainable phenomenon. Source: Pixabay.

The O ₂ is also responsible for the existence of fire: without it it would be almost impossible for there to be fires and combustion. This is because its main property is that of being a powerful oxidizing agent, gaining electrons or reducing itself in a water molecule, or in oxide anions, O ^2-.

Molecular oxygen is essential for countless aerobic processes, having applications in metallurgy, medicine, and wastewater treatment. This gas is practically synonymous with heat, respiration, oxidation and, on the other hand, with freezing temperatures when it is in its liquid state.

Structure of molecular oxygen

Molecular structure of gaseous oxygen. Source: Benjah-bmm27 via Wikipedia.

In the upper image we have the molecular structure of gaseous oxygen represented with various models. The last two show the characteristics of the covalent bond that holds the oxygen atoms together: a double bond O = O, in which each oxygen atom completes its valence octet.

The O ₂ molecule is linear, homonuclear, and symmetric. Its double bond has a length of 121 pm. This short distance means that some considerable energy (498 kJ / mol) is required to break the O = O bond, and therefore it is a relatively stable molecule.

If not, the oxygen in the atmosphere would have completely degraded over time, or the air would catch fire out of nowhere.

Properties

Physical appearance

Molecular oxygen is a colorless, tasteless and odorless gas, but when it condenses and crystallizes, it acquires bluish tones.

Molar mass

32 g / mol (rounded value)

Melting point

-218 ºC

Boiling point

-183

Solubility

Molecular oxygen is poorly soluble in water, but sufficient to support marine fauna. If your solubility were higher, you would be less likely to die from drowning. On the other hand, its solubility is much higher in nonpolar oils and liquids, being able to slowly oxidize them and thus affect their original properties.

Energy states

Molecular oxygen is a substance that cannot be fully described by valence bond theory (VTE).

The electronic configuration of oxygen is as follows:

2s² 2p⁴

It has one pair of unpaired electrons (O:). When two oxygen atoms meet, they bond to form an O = O double bond, both completing the valence octet.

Therefore, the O ₂ molecule should be diamagnetic, with all its electrons paired. However, it is a paramagnetic molecule, and this is explained by the diagram of its molecular orbitals:

Molecular orbital diagram for oxygen gas. Source: Anthony.Sebastian / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)

Thus, the molecular orbital theory (TOM) best describes O ₂. The two unpaired electrons are located in the higher-energy π ^* molecular orbitals, and give oxygen its paramagnetic character.

In fact, this energetic state corresponds to triplet oxygen, ³ O ₂, the most predominant of all. The other energy state of oxygen, less abundant on Earth, is singlet, ¹ O ₂.

Transformations

Molecular oxygen is considerably stable as long as it is not in contact with any substance that is susceptible to oxidization, much less if there is no nearby source of intense heat, such as a spark. This is because O ₂ has a high tendency to reduce itself, gain electrons from other atoms or molecules.

When reduced, it is able to establish a wide spectrum of links and shapes. If it forms covalent bonds, it will do so with atoms less electronegative than itself, including hydrogen, to give rise to water, HOH. It can also universe carbon, to originate CO bonds and various types of oxygenated organic molecules (ethers, ketones, aldehydes, etc.).

O ₂ can also gain electrons to transform into peroxide and superoxide anions, O ₂ ^2- and O ₂ ^-, respectively. When it is converted into peroxide within the body, hydrogen peroxide, H ₂ O ₂, HOOH, is obtained, a harmful compound that is processed by the action of specific enzymes (peroxidases and catalases).

On the other hand, and no less important, O ₂ reacts with inorganic matter to become the oxide anion, O ^2-, making up an endless list of mineralogical masses that thicken the earth's crust and mantle.

Applications

Welding and combustion

Oxygen is used to burn acetylene and give off an extremely hot flame that is valuable in welding. Source: Sheila / CC BY (https://creativecommons.org/licenses/by/2.0)

Oxygen is used to carry out the combustion reaction, by which a substance is exothermically oxidized, giving off fire. This fire and its temperature vary depending on the substance that is burning. Thus, very hot flames, such as acetylene (above), can be obtained with which metals and alloys are welded.

If not for oxygen, fuels could not burn and provide all their caloric energy, used to launch rockets, or to start cars.

Oxidizing agent in green chemistry

Thanks to this gas, a myriad of organic and inorganic oxides are synthesized or industrially produced. These reactions are based on the oxidizing power of molecular oxygen, being also one of the most viable reagents in green chemistry for obtaining pharmaceutical products.

Assisted breathing and wastewater treatment

Oxygen is vital to meet the respiratory demand in patients with serious health conditions, in divers when descending to shallow depths, and in mountain climbers, at whose altitudes the oxygen concentration is dramatically reduced.

Also, oxygen "feeds" aerobic bacteria, which help to break down polluting residues from sewage, or help fish breathe, in aqueous cultures for protection or trade.

References

1. Shiver & Atkins. (2008). Inorganic chemistry. (fourth edition). Mc Graw Hill.
2. Wikipedia. (2020). Allotropes of oxygen. Recovered from: en.wikipedia.org
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4. Kevin Beck. (January 28, 2020). 10 Uses for Oxygen. Recovered from: sciencing.com
5. Cliffsnotes. (2020). Biochemistry I: The Chemistry of Molecular Oxygen. Recovered from: cliffsnotes.com
6. GZ Industrial Supplies. (2020). Industrial benefits of Oxygen gas. Recovered from: gz-supplies.com