Reactions of metals with oxygen in air

Many metals react with oxygen to form metal oxides. Potassium and sodium are soft metal which are easily cut exposing a shiny surface which changes to dull rapidly. The change from shiny to dull is called tarnishing.

For example:

Potassium burns with a lilac flame when heated in air.

potassium + oxygen ➞ potassium oxide

4K(s) + O2(g) ➞ 2K2O(s)

Magnesium reacts readily in air burning with a white light.

magnesium + oxygen ➞ magnesium oxide

2Mg(s) + O2(g) → 2MgO(s)

Magnesium ribbon burning
Magnesium reacts vigorously with oxygen to produce magnesium oxide