Construct the nh4+ lewis structure for the ammonium ion, then determine (1) the formal charge on nitrogen and (2) the molecular geometry around nitrogen.
Step 1: Count total valence electrons
For a polyatomic ion, we start with the valence contribution of each atom and adjust for the overall charge.
| Species | Count | Valence e⁻ Each | Subtotal |
|---|---|---|---|
| N | 1 | 5 | 5 |
| H | 4 | 1 | 4 |
| Charge (+1) | Subtract 1 electron | -1 | |
| Total Sum | 8 | ||
Key Takeaway: With only 8 valence electrons, the structure consists of 4 single bonds (N–H) and no lone pairs on the central nitrogen.
Step 2 & 3: Skeleton and Bond Distribution
Nitrogen acts as the central atom. Connecting four hydrogens to it using single bonds accounts for all 8 electrons, satisfying nitrogen's octet and hydrogen's duet.
Step 4: Formal Charges
The total charge of the ion is +1. Calculating the formal charge for each atom verifies where that charge resides.
| Atom | Valence (V) | Lone (L) | Bond (B) | FC = V - L - B/2 |
|---|---|---|---|---|
| N | 5 | 0 | 8 | 5 - 0 - 4 = +1 |
| H | 1 | 0 | 2 | 1 - 0 - 1 = 0 |
Step 5: Visualization & Geometry
According to VSEPR theory, four electron domains arrange themselves in a Tetrahedral geometry to minimize repulsion.
Summary
- Valence Electrons: 8 (5 from N, 4 from H, -1 for charge)
- Formal Charge: +1 on the central Nitrogen atom
- Molecular Geometry: Tetrahedral
- Hybridization: sp³