What is the formal charge on the central nitrogen atom in dinitrogen sulfide (N2S)? a. –2 b. +2 c. –1 d. +1 e. 0
The correct answer and explanation is:
Correct answer: d. +1
To determine the formal charge on the central nitrogen atom in dinitrogen sulfide (N₂S), we follow the formal charge formula:
Formal charge = (Valence electrons) – (Non-bonding electrons) – (Bonding electrons ÷ 2)
Step 1: Determine the molecular structure of N₂S
Dinitrogen sulfide has the molecular formula N–N=S, and the common Lewis structure places the atoms in a linear or bent arrangement:
N₁–N₂=S
Let us consider a likely resonance form that satisfies the octet rule for all atoms:
:N≡N–S:
In this structure:
- The first nitrogen (N₁) has a triple bond with the second nitrogen (N₂) and one lone pair.
- The central nitrogen (N₂) has a single bond with sulfur and a triple bond with N₁.
- The sulfur atom (S) has two lone pairs and a single bond to N₂.
Step 2: Count formal charges
Valence electrons:
- Nitrogen has 5 valence electrons.
- Sulfur has 6 valence electrons.
Central nitrogen (N₂):
- Valence electrons = 5
- Non-bonding electrons = 0 (no lone pairs on N₂)
- Bonding electrons = 8 (3 bonds to N₁ + 1 bond to S = 4 bonds = 8 electrons)
Formal charge = 5 – 0 – (8 ÷ 2) = 5 – 0 – 4 = +1
Final Answer:
The formal charge on the central nitrogen atom in dinitrogen sulfide is +1.
Explanation:
Formal charges help chemists understand electron distribution in molecules. In N₂S, the central nitrogen forms four bonds but lacks non-bonded electrons, causing an electron deficiency. This is reflected as a formal charge of +1, indicating the atom has one less electron than it normally possesses in a neutral state. This helps balance the molecule’s overall charge and predict its reactivity.