Here's how to determine the formula of the ionic compound formed by sodium and sulfide ions:

1. Identify the charges:

* Sodium (Na) forms a +1 ion (Na⁺)

* Sulfide (S) forms a -2 ion (S²⁻)

2. Balance the charges:

* To achieve electrical neutrality, you need two sodium ions (2 x +1 = +2) to balance the charge of one sulfide ion (-2).

3. Write the formula:

The formula for the ionic compound is Na₂S.