Which of the following best describes the Lewis dot structure for the NH_2^- ion?

• A. Nitrogen is bonded to three hydrogens, has one lone pair, and carries a negative charge.
• B. Nitrogen is bonded to two hydrogens, has two lone pairs, and carries a negative charge.
• C. Nitrogen is bonded to two hydrogens, has one lone pair, and carries a positive charge.
• D. Nitrogen is bonded to two hydrogens, has no lone pairs, and carries a negative charge.

Answer: (B)

Verified step by step guidance

1. Step 1: Determine the total number of valence electrons for the NH_2^- ion. Nitrogen has 5 valence electrons, each hydrogen has 1 valence electron, and the negative charge adds 1 extra electron. So, total valence electrons = 5 + (2 × 1) + 1.
2. Step 2: Draw the skeletal structure with nitrogen as the central atom bonded to two hydrogen atoms. Connect each hydrogen to nitrogen with a single bond, which uses 2 electrons per bond.
3. Step 3: Subtract the electrons used in bonding from the total valence electrons to find the remaining electrons to be placed as lone pairs.
4. Step 4: Place the remaining electrons as lone pairs on the nitrogen atom. Remember that nitrogen typically follows the octet rule, so it should have a total of 8 electrons around it (bonding + lone pairs).
5. Step 5: Assign the formal charge to nitrogen by using the formula: Formal charge = (Valence electrons of atom) - (Nonbonding electrons) - (Bonding electrons / 2). Confirm that the nitrogen carries the negative charge, consistent with the ion's overall charge.