Oxygen, which has atomic number 8, belongs to the 16th group, p-block, of the periodic table (also known as chalcogens). It is the second most abundant gas in the Earth’s atmosphere. Diatomic oxygen contributes to about 21% of our atmosphere. Dioxygen (O2) is the most common allotrope of oxygen. Its other names include molecular oxygen and oxygen gas.

It is one of the most electronegative elements and is highly reactive. It readily reacts with almost all metals and non-metals except noble metals like gold (Au), platinum (Pt), and noble gases. The compounds so formed are known as oxides. Almost half the Earth’s crust is made up of oxygen in the form of oxides. Let us study in detail the methods of preparation, physical and chemical properties of dioxygen, and its uses.

Occurrence

On the Earth, oxygen is the most abundant element by mass. It is also the third most common in the universe (after hydrogen and helium). 

It makes up 49.2% of the Earth’s crust. Elemental oxygen is rarely found as it is a highly reactive gas and forms compounds with nearly all the elements.

Methods of preparation 

Laboratory methods

1. Dioxygen can be prepared by decomposing salts that contain oxygen, like chlorates, nitrates, and permanganates.

2 KClO3 → 2 KCl + 3O2 

The catalyst used here is manganese dioxide (MnO2), and the temperature is 420K.

2 KMnO4 → K2MnO4 + MnO2 + O2 

Heating potassium permanganate results in the formation of potassium manganate, manganese dioxide, and oxygen gas.

2 KNO3 → 2 KNO2 + 3O2

BaO2 → Ba + O2

2 NaNO3 → 2 NaNO2 + O2

1. The thermal decomposition of metal oxides with low electrode potential can lead to the formation of dioxygen gas.

2 HgO (s) → 2 Hg (l) + O2 (g)

2 Ag2O (s) → 4 Ag (l) + O2 (g)

1. Dioxygen can also be formed by heating higher oxides of metals like lead.

2 PbO2 (s) → 2 PbO (s) + O2 (g)

2 Pb3O4 (s) → 6 PbO (s) + O2 (g)

1. Another way of producing dioxygen is by the action of water on sodium peroxide (Na2O2)

2 Na2O2 + 2H2O → 4 NaOH + O2 (g)

1. The decomposition of hydrogen peroxide (H2O2) causes the release of dioxygen and water. Manganese dioxide (MnO2) is used to increase the rate of reaction.

2 H2O2 (aq) → 2 H2O (l) + O2 (g)

Industrial methods

1. Fractional distillation of liquified air can separate oxygen. First, carbon dioxide and water are removed. Then nitrogen gas (N2) is distilled as a vapour, and oxygen is left as a liquid. 
2. Dioxygen can also be obtained by passing a clean, dry air stream through a zeolite molecular sieve. N2 gas is absorbed, and the gas delivered contains 90%-93% oxygen. 
3. Electrolysis of water also yields oxygen. Hydrogen is released at the cathode while oxygen is liberated at the anode.

Structure

In the diatomic oxygen molecule, two atoms are covalently bonded to each other. The bond length of the O2 molecule is 121 pm, and the bond energy is 498 kJ/mol.

It is paramagnetic because of the presence of two unpaired electrons.

Physical properties of dioxygen

1. Its molecular mass is 31.99 g/mol. For convenience, a molecular mass of 32 g/mol is used.
2. It is a colourless gas and has no smell.
3. It exists as a slightly bluish liquid at -183 °C.
4. It is highly soluble in water (3.08 cm3 in 100 cm3 of water at 293K).
5. It has a boiling point of −182.962 °C.
6. Its melting point is −218.79 °C.
7. Its density is 1.429 g/L, and it is heavier than air.
8. It is paramagnetic.

Chemical properties of dioxygen

1. After fluorine, dioxygen is the second most electronegative element of the periodic table.
2. Its reaction with other elements is exothermic.
3. The solubility of oxygen is higher in cold water than in warm water.
4. Pure oxygen is toxic for human beings.

• Reaction with metals

1. M + O2 → MO2

K + O2 → KO2

1. 4 M + O2 → 2 M2O

4 Na + O2 → 2 Na2O

1. 4 M + 3 O2 → 2 M2O3

4 Al + 3 O2 → 2 Al2O3

2. Reaction with non-metals 

1. C + O2 → CO2 
2. P4 + 5 O2 → P4O10

3. Reaction with compounds

1. 2 ZnS + 3 O2 → 2 ZnO + 2 SO2
2. 2 SO2 + O2 → 2 SO3

Vanadium pentoxide (V2O5) is used as a catalyst here.

1. CH4 + 2 O2 → CO2 + 2 H2O
2. C6H12O6 + 6 O2 → 6 CO2 + 6 H2O

Carbon dioxide and water are formed when oxygen reacts with hydrocarbons and carbohydrates.

1. 4 HCl + O2 → 2 Cl2 + 2 H2O

CuCl2 is used as a catalyst to increase the rate of reaction.

4. The action of silent electric discharge

Oxygen gas is converted into ozone when treated with electricity.

3 O2 → 2 O3

Anomalous behaviour of oxygen

With respect to the other elements of group 16, oxygen shows variations in many of the properties.

1. It exists as a diatomic molecule while other chalcogens are octatomic.
2. It is the only chalcogen to form hydrogen bonds. 
3. It also shows pπ-pπ bonding due to its small size and high electronegativity.

Uses of dioxygen

1. It is used to synthesize ammonia, methyl alcohol, acetylene, etc.
2. It is used to enrich air for combustion in blast furnaces and flash smelters for nickel and copper.
3. Liquid oxygen is used in rocket propellants.
4. It is important for welding metals.
5. Oxygen supplementation is used in medicine and the manufacturing of medicinal drugs.
6. Oxygen therapy treats various heart disorders, pulmonary disorders, and any disease where the body cannot use gaseous oxygen.
7. Increasing the partial pressure of oxygen kills anaerobic bacteria.
8. It finds use as a low-pressure breathing gas in modern spacesuits.
9. Artificially delivered oxygen is essential for scuba divers.
10. It is widely used in the production of acrylic acid.
11. Mountaineers use oxygen cylinders.
12. It is also used to treat water.
13. Dioxygen is used in laser cutting.

Conclusion

Oxygen is a highly reactive colourless, odourless, tasteless gas. It is not found in its pure state on the Earth’s crust. Due to its high electronegativity and reactivity, it forms oxides with almost all metals and non-metals. It can be prepared by heating and decomposing oxygen-rich compounds like potassium permanganate, potassium chlorate, potassium nitrate, barium peroxide, etc. Oxygen can also be obtained by peroxides and water (through electrolysis).

Dioxygen is paramagnetic and supports combustion. It shows hydrogen bonding.

Oxygen is an essential element for the continuity of life on earth. It is the centre of the respiration process. It is used to fill artificial respiratory tanks in hospitals, mountaineering, and scuba diving. There are plenty of commercial uses of oxygen. It plays an important role in the welding and manufacturing of many metals like steel.