The nitrite ion ( ext{NO}_2^-) has a total of 18 valence electrons. To determine this, we must consider the contributions from each atom in the ion.
- Nitrogen (N) has 5 valence electrons.
- Each Oxygen (O) atom has 6 valence electrons, and since there are two oxygen atoms, that accounts for 2 × 6 = 12 valence electrons.
- Since the nitrite ion has a negative charge of -1, we must add one additional electron.
Adding these together: 5 (from N) + 12 (from O) + 1 (for the negative charge) = 18 valence electrons.
Therefore, the correct answer is d) 18.