We had a test with a question asking for the number of valence electrons in an azide ion, $\text{N}_3^{-}$. I found out that the answer is 16, but I could not understand how.

I saw the structure, and it states that one of the end nitrogens has a -1 charge, while the middle nitrogen has a +1 charge. From where does one of the end nitrogens get the extra electron, assuming the other end nitrogen gets it from the middle nitrogen?

The azide ion, N3-, contains a triple bond between one nitrogen atom and another nitrogen atom. The remaining lone pair of electrons on each nitrogen atom gives each end nitrogen atom a formal charge of -1. The middle nitrogen atom, which is in the center of the ion, has a formal charge of +1. The total number of valence electrons in the azide ion is 16.

Explanation:

The azide ion, N3-, contains a triple bond between one nitrogen atom and another nitrogen atom. The remaining lone pair of electrons on each nitrogen atom gives each end nitrogen atom a formal charge of -1. The middle nitrogen atom, which is in the center of the ion, has a formal charge of +1. To determine the number of valence electrons in the azide ion, we add up the valence electrons on each nitrogen atom and the charge on the ion. Since each nitrogen atom has 5 valence electrons and the ion has a charge of -1, the total number of valence electrons in the azide ion is 16.