#Na_3N#. Sodium is an element in group 1 of the periodic table. As such, it will form an electron by losing a single electron to form a #1+# ion: #Na^+#. Nitrogen is an element in group 5 of the periodic table. When it forms the nitride ion, it gains three electrons to form a #3-# ion: #N^(3-)#. In ionic compounds, the charges of constituent ions must balance. This can be achieved by having three sodium ions per nitride ion. Therefore, the formula of sodium nitride is #Na_3N#.