Final answer:
The charge on the nitride ion is -3, which is derived from nitrogen's position in group 15 of the periodic table.
Step-by-step explanation:
The charge on the nitride ion is -3. This falls under the rule where the charge on monoatomic ions can be determined by the group number minus eighteen. Because nitrogen is in group 15 of the periodic table, its charge is calculated as 15-18, resulting in a charge of -3.