Oxygen and Air

Earth’s atmosphere is a gaseous mixture essential for sustaining life and driving geochemical cycles. In environmental chemistry, dry air at sea level is treated as a standard reference mixture composed primarily of non-metals.

Component	Chemical Formula	Percentage by Volume	Environmental & Chemical Role
Nitrogen	N2	~78.08%	Chemically inert at room temperature due to a strong triple bond (N≡N); dilutes oxygen to prevent rapid, uncontrollable combustion.
Oxygen	O2	~20.95%	Highly reactive non-metal; essential for respiration, combustion, and chemical oxidation.
Argon	Ar	~0.93%	Noble gas; chemically unreactive; used in industrial applications requiring an inert atmosphere.
Carbon Dioxide	CO2	~0.04%	Core greenhouse gas; vital for plant photosynthesis; acts as a primary driver of global climate regulation.
Trace Gases	Ne, He, CH4, Kr	<0.04%	Includes neon, helium, methane, and krypton; variable concentrations depending on local industrial activity.

Chemical Properties of Oxygen

Oxygen (Atomic Number, Z = 8) has an electronic configuration of 1s² 2s² 2p⁴. To achieve a stable octet, it readily gains or shares two electrons, making it highly electronegative (second only to fluorine). Elementally, it exists as a colorless, odorless, and tasteless diatomic gas (O₂). While O₂ is non-flammable on its own, it is a powerful oxidant that vigorously supports combustion. It dissolves moderately in water (~30.8 mg/L at 0°C), a property critical for the survival of aquatic organisms.

Allotropy of Oxygen: Dioxygen vs. Ozone

Oxygen exists in two primary allotropic forms in the atmosphere: Dioxygen (O₂) and Ozone (O₃). These allotropes display distinct physical and environmental characteristics.

Dioxygen (O₂)

Dioxygen is the standard, stable molecular form of oxygen found in the troposphere. It is paramagnetic in nature due to the presence of two unpaired electrons in its molecular orbitals, a feature that influences its quantum chemical interactions during combustion and enzymatic oxidation.

Ozone (O₃)

Ozone is a pale blue, toxic gas with a pungent odor. Structurally, it is a bent molecule exhibiting resonance stabilization. Unlike O₂, ozone is diamagnetic and acts as a far more potent oxidizing agent.

• The Chapman Cycle (Stratospheric Ozone Formation): In the stratosphere, ozone is continuously formed and destroyed via a photochemical cycle driven by ultraviolet (UV) radiation.
  1. Photolysis of O₂ by high-energy UV-C light:
     O₂ + hν (UV-C) → O + O
  2. Combination of atomic oxygen with molecular oxygen:
     O + O₂ + M → O₃ + M (where M is a catalytic third body like N₂)
  3. Photodissociation of ozone by UV-B light:
     O₃ + hν (UV-B) → O₂ + O

Environmental Chemistry: Oxidation, Corrosion, and Pollution

Oxides and Chemical Classification

Oxygen reacts with almost all elements (except noble gases) to form oxides. These are categorized based on their acid-base behavior when dissolved in water:

• Acidic Oxides: Formed primarily by non-metals. They react with water to produce acids. Example: Carbon dioxide (CO₂), Sulfur dioxide (SO₂), and Nitrogen dioxide (NO₂).
  SO₂ + H₂O → H₂SO₃ (Sulfurous Acid)
• Basic Oxides: Formed primarily by metals. They react with water to form bases. Example: Sodium oxide (Na₂O) and Calcium oxide (CaO).
  CaO + H₂O → Ca(OH)₂ (Calcium Hydroxide)
• Amphoteric Oxides: Exhibit both acidic and basic properties. Example: Aluminum oxide (Al₂O₃) and Zinc oxide (ZnO).
• Neutral Oxides: Do not display acidic or basic characteristics. Example: Carbon monoxide (CO), Nitrous oxide (N₂O), and Nitric oxide (NO).

Corrosion and Rusting

Rusting is an electrochemical phenomenon where iron reacts with oxygen and moisture in the air to form hydrated ferric oxide.

This process is accelerated by atmospheric pollutants like SO₂ and dissolved salts, which increase the electrical conductivity of the moisture layer on the metal surface.

Ground-Level Ozone vs. Stratospheric Ozone

In environmental chemistry, ozone is evaluated based on its atmospheric location:

• Good Ozone (Stratospheric): Located 15-30 km above Earth. It shields the biosphere by absorbing harmful solar UV-B and UV-C radiation.
• Bad Ozone (Tropospheric / Ground-Level): A secondary air pollutant formed at ground level. It is created by photochemical reactions between Volatile Organic Compounds (VOCs) and Nitrogen Oxides (NO_x) in the presence of sunlight. It is a primary component of photochemical smog, causing respiratory illnesses in humans and damaging agricultural crops.

Industrial Processes and Environmental Metrics

Dissolved Oxygen (DO)

Dissolved Oxygen refers to the volume of free, non-compound oxygen dissolved in water. It is a critical indicator of water quality and aquatic ecosystem health. Healthy water bodies typically maintain DO levels between 4-8 mg/L. If DO falls below 4 mg/L, aquatic organisms experience severe stress, leading to fish kills.

Biochemical Oxygen Demand (BOD)

BOD measures the amount of dissolved oxygen required by aerobic microorganisms to decompose organic matter present in a water sample over a specific period (usually 5 days at 20°C).

• High BOD indicates a large amount of organic pollution (typically from sewage or agricultural runoff).
• As BOD increases, the Dissolved Oxygen (DO) of the water body drops rapidly, causing a phenomenon known as hypoxia.

Chemical Oxygen Demand (COD)

COD measures the total quantity of oxygen required to chemically oxidize all organic and inorganic susceptible matter in a water sample using a strong chemical oxidant (like potassium dichromate, K₂Cr₂O₇ in acid).

• Unlike BOD, which only measures biologically degradable matter, COD measures both biodegradable and non-biodegradable organic matter.
• Key Fact: The COD value of a wastewater sample is always higher than its BOD value.

Industrial Applications of Oxygen

• Steel Manufacturing: The Basic Oxygen Steelmaking (BOS) process uses pure oxygen to oxidize impurities like carbon, silicon, and phosphorus out of liquid pig iron.
• Oxy-Acetylene Welding: Mixing pure oxygen with acetylene gas produces a flame temperature exceeding 3000°C, capable of cutting and welding heavy metals.
• Medical and Aerospace Support: Liquid oxygen (LOX) is used as an oxidant in cryogenic rocket engines (e.g., ISRO’s CE-20 engine) and as a life-support gas in aviation, space exploration, and healthcare emergency systems.

Last Modified: May 27, 2026