How can I write the formula for sodium sulfide?

The formula for sodium sulfide is ##Na_2S##

. Since this is an ionic compound, you must balance the charges so that overall charge of the compound is neutral. Sodium, an alkali metal, has a tendency to lose one electron. As a result sodium normally carries a positive one charge. Sulfur, a nonmetal, has a tendency to gain 2 electrons. This results in an ion with a negative 2 charge. Nonmetal ions end in "ide". To obtain a neutral charge, you need two sodium ions, which provides you with a plus 2 charge balance the negative 2 charge of sulfur.

Therefore this is the chemical equation.