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.
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)