When sodium is heated in air, it burns with a golden yellow flame to form sodium peroxide $2N{a}_{(s)}+O_{2(g)}\to N{a}_2{O}_{2(s)}$
If it is exposed to air (without heating) it forms sodium oxide $\left(N{a}_{2}O\right)$. The moisture present in the air combines with some of the oxide to form the hydroxide and finally, after a time, the carbon dioxide of the air combines with the sodium hydroxide to form sodium carbonate which crystallizes to form the decahydrate.
$4N{a}_{(s)}+O_{2(s)}\to 2N{a}_2{O}_{(s)}$
$N{a}_2{O}_{(s)}+H_2{O}_{(n)}\to 2NaO{H}_{(aq)}$
$2NaO{H}_{(aq)}+C{O}_{2(g)}\to N{a}_{2}C{O}_{3(s)}+H_{2}O$
$N{a}_{2}C{O}_{3(s)}+10H_2{O}_{(0)}\to N{a}_{2}C{O}_3,10H_2{O}_{(s)}$