Final answer:
The formula of the nitride ion is N₃⁻, as it gains three electrons to fill its outer shell, giving it a – 3 charge. When forming ionic compounds like calcium nitrate, we use parentheses to balance the charges in formulas as demonstrated in Ca(NO₃)₂.
Step-by-step explanation:
The formula of the nitride ion is N₃⁻. This is because the element nitrogen, when forming an ion, typically gains three electrons to achieve a full octet, corresponding to the nearest noble gas configuration, which is neon in this case. As a result, nitrogen has a – 3 charge as an ion, making the correct answer c. N₃⁻.
For writing formulas of ionic compounds, it's important to balance the overall charge. If we look at examples such as calcium nitrate, we see that the compound is composed of Ca²⁺ and NO₃⁻ ions. The correct formula is Ca(NO₃)₂ to balance the charges, with parentheses used around the nitrate ion to indicate that two nitrate ions are needed for each calcium ion.