Oxygen (O) — Atomic Number 8

Oxygen is a chemical element with the symbol O and atomic number 8. It is a highly reactive non-metal and powerful oxidising agent. It is the most abundant element in Earth’s crust (~46%) and makes up 21% of the atmosphere. The name comes from Greek oxys (“acid”) + genes (“former”) — Lavoisier named it believing all acids contained oxygen.

At a Glance

Discovery

Independently discovered by Carl Scheele (Sweden, 1771) and Joseph Priestley (England, 1774). Antoine Lavoisier named it and demonstrated its role in combustion, overturning the phlogiston theory.

Allotropes

• Dioxygen (O₂): The common form. Double bond (O=O). Makes up 21% of air.
• Ozone (O₃): Triatomic, pale blue, sharp odour. In the stratosphere, it absorbs harmful UV radiation. At ground level, it is a pollutant (photochemical smog).

Chemical Properties

Oxygen is a powerful oxidising agent — it supports combustion but does not burn.

Key Reactions

1. With magnesium: $2\text{Mg}+{\text{O}}_2\to 2\text{MgO}$ (brilliant white light)
2. With carbon: $\text{C}+{\text{O}}_2\to {\text{CO}}_2$
3. With sulfur: $\text{S}+{\text{O}}_2\to {\text{SO}}_2$ (blue flame, choking gas)
4. Rusting: $4\text{Fe}+3{\text{O}}_2+x{\text{H}}_2\text{O}\to 2{\text{Fe}}_2{\text{O}}_3\cdot x{\text{H}}_2\text{O}$ (requires both O₂ AND water)
5. Respiration: ${\text{C}}_6{\text{H}}_{12}{\text{O}}_6+6{\text{O}}_2\to 6{\text{CO}}_2+6{\text{H}}_2\text{O}+\text{energy}$

Characteristic Test

Relights a glowing splint

Lab Preparation

Catalytic decomposition of hydrogen peroxide: $2{\text{H}}_2{\text{O}}_2\stackrel{{\text{MnO}}_2}{\to }2{\text{H}}_2\text{O}+{\text{O}}_2$

IGCSE Essentials

1. Test: relights a glowing splint
2. Supports combustion — not flammable itself
3. 21% of air
4. Rusting needs O₂ AND water
5. Metal + O₂ → basic oxide; non-metal + O₂ → acidic oxide

Sources

• Oxygen — Wikipedia
• Oxygen — Ptable.com
• Oxygen — RSC Periodic Table