What happens when metal reacts with oxygen?

When a metal reacts with oxygen metallic oxide is formed. Many metals react with oxygen to form metal oxides. K and Na are soft metals that are easily cut exposing a shiny surface that changes to dull rapidly.

• Potassium burns with a lilac flame when heated in the air.
• potassium + oxygen $\to$potassium oxide
  $4K\left(s\right)+O_2\left(g\right)\to 2{\left(K\right)}_2{O}_{\left(s\right)}$
• Magnesium reacts readily in the air burning with white light.
• Magnesium $+$ oxygen $\to$ magnesium oxide 
  $2Mg\left(s\right)+O_2\left(g\right)\to 2MgO\left(s\right)$