Final answer:
The formula of the nitride ion is N³⁻, which represents a nitrogen atom that has gained three electrons, thus having a -3 charge.
Step-by-step explanation:
The formula of the nitride ion is N³⁻. Nitrogen, being a nonmetal located in group 15 of the periodic table, forms anions by gaining electrons to achieve a noble gas electron configuration. Specifically, a nitrogen atom needs to gain three electrons to fill its valence shell like the noble gas neon, resulting in a triply-charged negative ion. Therefore, the symbol for this ion is N³⁻, which denotes a nitrogen atom with three extra electrons, carrying a -3 charge.