Reactions of alkali metals with oxygen

When the alkali metals are cut, they initially appear shiny grey but quickly become dull and white as they react with oxygen in the air. This is known as tarnishing.

Lithium

Lithium tarnishes slowly due to its relatively slow reaction with oxygen.

lithium + oxygen → lithium oxide

4Li(s) + O2(g) → 2Li2O(s)

Sodium

Sodium tarnishes more quickly than lithium, which is further evidence for the greater reactivity of sodium when compared to lithium.

sodium + oxygen → sodium oxide

4Na(s) + O2(g) → 2Na2O(s)

Potassium

Potassium tarnishes so quickly that it is difficult to see that potassium is actually a shiny metal. This is further evidence that potassium is a more reactive metal than both lithium and sodium.

potassium + oxygen → potassium oxide

4K(s) + O2(g) → 2K2O(s)

Combustion

The alkali metals can also be set alight and burn. When any substance burns in oxygen it is called a combustion reaction. Potassium (lilac) burns most vigorously followed by sodium (orange-yellow) and then lithium (red), as you might expect.

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The word and symbol equations for the combustion reactions of the alkali metals are exactly the same as the equations for tarnishing as they are both reactions of the alkali metals with oxygen.