Sodium more reactive metal, it can easily react with oxygen and form oxide to prevent this we have to kept it under kerosene.

Sodium can easily react with oxygen and moisture and forms sodium oxide with liberation of ${\text{H}}_{\text{2}}$ gas.  This reaction is an explosive reaction because of high reactivity of sodium.  So Na metal should be stored in kerosene.

${\text{2Na + H}}_{\text{2}}{\text{O}}_{\left(\text{Cold}\right)}\to {\text{Na}}_{\text{2}}\text{O}+{\text{H}}_{\text{2}}\left(g\right)\uparrow$