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Well, you know that Al gives a stable trivalent cation, so in a compound with the general formula AlX, X must be a trivalent anion.

Oxygen and sulfur both give divalent anions at most (oxides and sulfides), and that'd be Al2X3

Fluorine and chlorine give monovalent anions (fluorides and chlorides), yielding AlX3.

Only nitrogen can form trivalent anions (nitrides), thus making a compound of the formula AlX possible.

Sulfur could've been discarted right away, since it's a solid at room temperature, and not a diatomic gas.

I don't have the same assignment as you, but I can answer this question.

D) Nitrogen.

Because sulfur is not a diatomic gas at room temperature. Chlorine and fluorine have a valence of -1, oxygen has a valence of -2, so neither of these could combine with aluminum (valence 3+) in a 1:1 ratio (indicated by the general formula AlX).

Nitrogen can have a valence of -3 (for example, ammonia is NH3) so it could react with aluminum to form AlN.

If it is diatomic, it would have electron levels that would have to cancel out with the Aluminum. If aluminum is +4, the diatomic molecule of gas would have to be O2, because Oxygen has a -2

charge, and two -2 equals -4 which cancels with the + 4 from the Aluminum. Hope this helped. :)

i've got not got the same project as you, yet i can respond to this question. D) Nitrogen. because of the fact sulfur isn't a diatomic gas at room temperature. Chlorine and fluorine have a valence of -a million, oxygen has a valence of -2, so neither of those ought to combine with aluminum (valence 3+) in a a million:a million ratio (indicated by skill of the final formula AlX). Nitrogen could have a valence of -3 (to illustrate, ammonia is NH3) so it ought to react with aluminum to kind AlN.

Al goes to Al(-3). You need a +3 counterion - the pnictide.