Here's why:

* Sodium (Na) is a metal in Group 1 of the periodic table, meaning it has a +1 charge when it forms an ion (Na⁺).

* Oxygen (O) is a nonmetal in Group 16, meaning it has a -2 charge when it forms an ion (O²⁻).

To form a neutral compound, you need two sodium ions (2 x +1 = +2) to balance out the charge of one oxide ion (-2).