Reactivity of Metals with Air

1. Introduction

Metals react differently with air depending on their position in the reactivity series. The reaction usually forms the metal oxide. Metals are classified as active or inactive based on their tendency to react with oxygen.

2. Active Metals

Active metals react readily with oxygen in air, often forming oxides quickly at room temperature or on heating.

Metal	Reaction with Air	Equation
Sodium (Na)	Ignites spontaneously in air forming white sodium oxide.	4Na + O2 → 2Na2O
Potassium (K)	Ignites even in cold air forming potassium peroxide.	K + O2 → K2O2
Calcium (Ca)	Burns in air with red flame forming calcium oxide.	2Ca + O2 → 2CaO
Magnesium (Mg)	Burns with bright white flame forming magnesium oxide.	2Mg + O2 → 2MgO

Observation:

• Active metals react vigorously, often producing heat and light.
• The oxide layer may prevent further reaction in some cases (passivation), but for very active metals, reaction continues.

3. Inactive Metals

Inactive metals react slowly or not at all with air at room temperature. High temperature may be required.

Metal	Reaction with Air	Equation
Iron (Fe)	Slowly reacts forming reddish-brown iron(III) oxide (rust).	4Fe + 3O2 → 2Fe2O3
Copper (Cu)	Does not react at room temperature. On heating, forms black copper(II) oxide.	2Cu + O2 → 2CuO
Silver (Ag)	Does not react under normal conditions; tarnishes on prolonged exposure forming Ag2O.	4Ag + O2 → 2Ag2O
Gold (Au)	Does not react with air; extremely unreactive.	—