The formula for the nitride ion is N³⁻.
Explanation: The nitride ion is formed when a nitrogen atom gains three electrons. Nitrogen is in group 15 of the periodic table and has five valence electrons. By gaining three additional electrons, it achieves a stable octet configuration, resembling the electron arrangement of the noble gas neon. The resulting ion has a negative charge of three because there are three more electrons than protons in the nitrogen atom. Therefore, the chemical formula for the nitride ion is represented as N³⁻, indicating it carries a -3 charge.