The Bond Angle of the H₃⁺ Ion
The H₃⁺ ion (trihydrogen cation) is the most abundant ion in the universe and plays a critical role in interstellar chemistry. It possesses a trigonal planar geometry with D₃ₕ symmetry.
In this structure, the three hydrogen nuclei (protons) form a perfect equilateral triangle. Because it is an equilateral triangle with no central atom, the H–H–H bond angle is exactly 60°.
Explanation
- Three-Center Two-Electron (3c-2e) Bond: The ion consists of three protons shared by only two electrons. These electrons are delocalized across all three atoms, creating a stable, cyclic structure.
- Bond Length: All H–H bond lengths are equal, measuring approximately 0.90 Å.
- The "120° Confusion": While "trigonal planar" molecules with a central atom (like BF₃) have 120° angles, H₃⁺ is a ring system. In any equilateral triangle, the internal angles must sum to 180°, making each angle 60°.
Why isn't H₃⁺ linear?
Quantum mechanics explains this through Molecular Orbital Theory. The two electrons in the system occupy a highly stable bonding orbital created by the overlap of all three 1s orbitals.
- Maximum Overlap: The triangular (cyclic) arrangement allows for better orbital overlap compared to a linear structure.
- Delocalization: The 2 electrons are shared across all 3 nuclei simultaneously, known as a three-center two-electron (3c-2e) bond.
- Bond Length: The H–H distance is approximately 0.90 Å.