3j. The covalent bonding of oxygen O₂ molecules

Covalent bonding diagram for OXYGEN covalent molecule, molecular formula O₂

 Two atoms of oxygen (2.6) combine to form the molecules of the element oxygen O₂ (only the outer shell of oxygen's electrons are shown).

Each oxygen atom is two electrons short of a full outer shell, so each oxygen atom shares two of its electrons with the other atom, so both oxygen atoms have a full outer shell.

(Lewis diagram of oxygen) simplified 'dot and cross' electronic diagram for the covalently bonded oxygen molecule

The diatomic molecule has one O=O double covalent bond (displayed formula), oxygen valency 2.

The oxygen molecule is held together by the strong O=O oxygen–oxygen double covalent bond by sharing electrons.

Electronically, by sharing two electrons, both oxygen atoms attain a pseudo neon structure (2.8), so the oxygen atoms effectively have full outer shells in forming the covalent bonds when the atoms share their outer electrons.

On the left is the full 'dot and cross' electronic diagram for the covalent bonding in the oxygen molecule showing the inner non-valence electrons - omitted from the right diagram.

The Venn style diagram above right is the best diagram for oxygen, clearly showing the sharing of two pairs of electrons for the double covalent bond in the oxygen molecule (in a sort of Venn diagram style).  The electronic dot & cross Lewis diagrams for covalent bonding in oxygen can be shown without the inner non-valency electrons (right diagram).

The ball and stick model of oxygen would be like chlorine with a = bond and coloured red. The space filling model would be identical.

Comments

Melting point of oxygen -218 ^oC

Boiling point of oxygen -189 ^oC

You would expect low values because of the weak intermolecular forces between small covalent molecules like oxygen.

The colourless odourless gas that we can't live without !!!

Extra notes on Oxygen (O₂) e.g. for GCSE/IGCSE Chemistry Revision Notes

Properties of oxygen

• Formula: O₂
• Molecular form: Diatomic molecule
• Physical properties:
  • Colourless, odourless gas
  • Slightly denser than air
  • Slightly soluble in water
• Chemical properties:
  • Supports combustion (oxidising agent)
  • Reacts with many elements to form oxides
  • Essential for respiration in living organisms

Reactions of oxygen (equations schematic, not balanced in symbols)

• Combustion:
  • Carbon + O₂ → CO₂
  • Hydrogen + O₂ → H₂O
  • Sulphur + O₂ → SO₂
• Formation of oxides:
  • Metals → basic oxides (e.g., 2Mg + O₂ → 2MgO)
  • Non-metals → acidic oxides (e.g., C + O₂ → CO₂)
• Respiration: Glucose + O₂ → CO₂ + H₂O + energy
• Oxidising reactions: Oxygen reacts with reducing agents (e.g., hydrogen, carbon monoxide).

Laboratory Preparation of oxygen

• Decomposition of hydrogen peroxide:
  2H₂O₂ → 2H₂O + O₂ (catalyst: MnO₂)
• Heating potassium manganate(VII):
  2KMnO₄ → K₂MnO₄ + MnO₂ + O₂
• Collection: Over water or by downward delivery (slightly denser than air).
• Test for oxygen: Relights a glowing splint.
• See also Methods of preparing and collecting gases revision notes
• and Tests for gases, water and non-metallic elements

Industrial Manufacture of oxygen

• Fractional distillation of liquid air:
  • Air cooled and liquefied
  • Oxygen separated (boiling point −183 °C)
• Electrolysis of water:
  2H₂O → 2H₂ + O₂ (less common, expensive)

Uses of Oxygen

• Medical: Breathing aids, oxygen therapy
• Industrial: Steel manufacture (blast furnaces), welding and cutting metals
• Environmental: Water treatment, combustion processes
• Everyday life: Respiration, combustion for energy
• Rocket fuel oxidiser: Liquid oxygen used with hydrogen

Typical Exam Board Requirements

Student Exam Tips

• Test for oxygen: Always state “relights a glowing splint” (not just “lights a splint”).
• Equations: Balance combustion equations carefully.
• Industrial manufacture: Fractional distillation of liquid air is the main method.
• Oxides: Distinguish between acidic oxides (non-metals) and basic oxides (metals).
• Exam wording: Oxygen is an oxidising agent — don’t confuse with “oxidation” meaning rusting only.
• Collection method: Over water or downward delivery (slightly denser than air).

Common Misconceptions

•  Thinking oxygen is flammable (it is not; it supports combustion).
•  Forgetting oxygen is diatomic (O₂, not O).
•  Confusing “oxidising agent” with “oxidation” — oxygen causes other substances to oxidise.
•  Assuming oxygen is the only component of air (air is ~21% oxygen, ~78% nitrogen).
•  Believing electrolysis is the main industrial method (fractional distillation of air is correct).