The answer if Na2O. Explanation: When you write the formula, write such a way that the compound is neutral. Since Na has 1+ charge and O has 2- charge, you need 2 Na to balance the 2- charge of O. Easy way to do this is to criss-cross the charge of the atoms as the subscripts in the formula. Use the simplest integer ratio at the end. Example: 1) Mg^2+ and P^3- Formula: Mg3P2 2) Ca2+ and O2- Formula: Do not write as Ca2O2. Instead write as CaO 3) Al^3+ and O^2- Formula: Al2O3.