When sodium (Na) reacts with oxygen (O2), it forms sodium oxide (Na2O). The chemical formula for sodium oxide is Na2O. This compound is an ionic compound, consisting of positively charged sodium ions (Na+) and negatively charged oxide ions (O2-). The reaction between sodium and oxygen can be represented by the following chemical equation:

4Na(s) + O2(g) → 2Na2O(s)

In this reaction, four atoms of sodium react with one molecule of oxygen to produce two molecules of sodium oxide.