The search phrase hcn lewis structure typically asks for the Lewis dot structure of hydrogen cyanide, HCN: the correct bonding (single vs. triple bond), the placement of lone pairs, and the formal charges.
Step 1: Count total valence electrons
Valence electrons come from the periodic table group numbers:
| Atom | Valence electrons per atom | Atoms present | Total contribution |
|---|---|---|---|
| H | 1 | 1 | \(1\) |
| C | 4 | 1 | \(4\) |
| N | 5 | 1 | \(5\) |
| Total | \(\,1 + 4 + 5 = 10\,\) valence electrons | ||
Step 2: Choose a reasonable skeleton
Hydrogen forms only one bond and cannot be central. Carbon is the most typical central atom in HCN, giving the skeleton: H–C–N.
Step 3: Add bonds, then complete octets
- Start with single bonds H–C and C–N. Two single bonds use \(2 \times 2 = 4\) electrons, leaving \(10 - 4 = 6\) electrons.
- Give the terminal atom (N) an octet first. With a single C–N bond, nitrogen would need three lone pairs (6 electrons) to reach an octet. That would use all remaining 6 electrons, but carbon would only have 4 electrons around it (not an octet).
- Convert lone pairs on nitrogen into bonding pairs between C and N until carbon achieves an octet. Converting two lone pairs into bonding pairs creates a triple bond C≡N.
After forming a triple bond, the structure becomes H–C≡N. The bond counts are: H–C is a single bond (2 electrons) and C≡N is a triple bond (6 electrons), for 8 bonding electrons total. One lone pair remains on N (2 electrons), giving the required total \(8 + 2 = 10\) valence electrons.
Step 4: Check formal charges
Formal charge verifies the best Lewis structure:
\[ \text{Formal charge} = \text{valence} - \left(\text{nonbonding} + \frac{\text{bonding}}{2}\right) \]
| Atom | Valence | Nonbonding electrons | Bonding electrons | Formal charge |
|---|---|---|---|---|
| H | \(1\) | \(0\) | \(2\) | \(1 - (0 + 2/2) = 0\) |
| C | \(4\) | \(0\) | \(8\) | \(4 - (0 + 8/2) = 0\) |
| N | \(5\) | \(2\) | \(6\) | \(5 - (2 + 6/2) = 0\) |
All formal charges are zero, so H–C≡N with one lone pair on N is the preferred Lewis structure of HCN (also called the HCN Lewis dot structure or Lewis structure HCN).
Visualization: Lewis dot structure and linear geometry
Final result
The Lewis structure of HCN is H–C≡N, with one lone pair on nitrogen, 10 total valence electrons, octets satisfied for C and N, and formal charges of 0 on H, C, and N.