What happens when you add the element sodium to water?

Sodium reacts violently with water.

2Na(s) + 2H2O(l) --> 2NaOH(aq) + H2(g) + heat

The heat of the reaction will ignite the H2 gas and make it explode, producing more heat.

H2(g) + O2(g) --> H2O(g) + heat

If you still have unreacted sodium, the heat from the explosion will cause sodium to ignite and explode.

Na(s) + O2(g) --> Na2O2(s)

Na2O2 is a peroxide that contains O22-, rather than the O2- ion you normally see.