The Lewis structure of HCN (H bonded to C) shows that N has _ bonding and __ nonbonding electron pair(s) respectively. A) 3, 1 B) 1, 1 C) 2, 2 D) 3, 2
The Correct Answer and Explanation is:
Correct Answer: A) 3, 1
Explanation:
The Lewis structure of HCN (hydrogen cyanide) provides a visual representation of how the atoms bond and how electrons are shared or remain unshared.
Hydrogen (H) has 1 valence electron and can form only one bond. Carbon (C) has 4 valence electrons and tends to form 4 bonds. Nitrogen (N) has 5 valence electrons and tends to form 3 bonds, often leaving one lone pair of electrons.
To construct the Lewis structure of HCN:
- Place hydrogen on one side, since it forms only one bond.
- Carbon is in the center because it forms the most bonds.
- Nitrogen is placed at the opposite end of carbon.
Now arrange the atoms as H–C≡N, with a triple bond between carbon and nitrogen, and a single bond between hydrogen and carbon.
Here’s the breakdown:
- Hydrogen shares its one electron with carbon, forming a single bond.
- Carbon uses three of its electrons to form a triple bond with nitrogen and one electron to bond with hydrogen.
- Nitrogen contributes three of its valence electrons to the triple bond with carbon. That makes 6 electrons in bonding pairs (3 bonding pairs).
- Nitrogen has 5 valence electrons in total. After donating 3 electrons for bonding, it has 2 left, which form one nonbonding pair (a lone pair).
Thus, nitrogen in HCN has:
- 3 bonding pairs of electrons shared with carbon (in the triple bond)
- 1 nonbonding pair (lone pair) of electrons that is not shared
This matches choice A: 3 bonding and 1 nonbonding pair. The structure satisfies the octet rule for carbon and nitrogen and the duet rule for hydrogen.
