An alkaline metal, element X, reacts with nitrogen. What is the general formula for the compound formed? A) XN₂, B) X₂N, C) X₂N₃, D) X₃N, E) X₃N₂.
In this question, we have an alkaline metal, which is clearly a metallic element, reacting with nitrogen, which is a nonmetallic element. As alkaline metals are located in group one of the periodic table, we can safely say that all alkaline metals have one electron in their valence shell. This valence shell electron is typically lost during a reaction with a nonmetal. This leaves an ion with a single positive charge and a stable electron configuration.
In a similar way, if the element X were to react with nitrogen, we would expect the ion X⁺ to be formed in the process. Having established that element X will form a stable X⁺ ion during the reaction with nitrogen, we need to turn our attention to the nitrogen atom now.
Nitrogen is a nonmetallic element in group 15 or group five of the periodic table. Nitrogen, therefore, contains five valence shell electrons in its atom. We can ignore the inner shell electrons because they’re not involved in this reaction. During the course of a reaction with a nonmetal, nitrogen can gain three electrons to form a stable nitride ion, N³⁻, with a full valence shell.
So now that we’ve established that X will form an X⁺ ion during this reaction and nitrogen will form an N³⁻ ion during the reaction, it becomes apparent that we’re dealing with an ionic compound. In ionic compounds, you typically find metals bonded to nonmetals via electrostatic attraction, which holds oppositely charged ions together in a lattice.
In a giant ionic lattice, the positive and negative ions are packed very closely together in a regular arrangement. The positive and negative charges must cancel each other out. All this to say there must be zero net charge overall. In our example here, we see that we need three positive charges to cancel off three negative charges located on the nitride ion. The only way to achieve this is to have three X⁺ ions reacting with one nitride ion.
Therefore, the ratio in which X⁺ ions react with N³⁻ ions must be three to one. So the general formula of our compound, which describes the actual ratio of atoms of each element in the compound, must therefore be X₃N. X₃N is the correct answer here.