According to the VSEPR theory what is the molecular geometry of H3O+ (hydronium ion)
The Correct Answer and Explanation is:
According to the Valence Shell Electron Pair Repulsion (VSEPR) theory, the molecular geometry of the hydronium ion (H₃O⁺) is trigonal pyramidal.
Explanation
The VSEPR theory is used to predict the shape of molecules based on the idea that electron pairs around a central atom will arrange themselves as far apart as possible to minimize repulsion. These pairs include both bonding pairs (shared between atoms) and lone pairs (non-bonding pairs localized on the central atom).
In the hydronium ion (H₃O⁺):
- The central atom is oxygen (O).
- It forms three single covalent bonds with three hydrogen atoms.
- Oxygen normally has six valence electrons, but due to the positive charge on H₃O⁺ (caused by gaining a proton, H⁺), there are only five electrons remaining around oxygen to use in bonding and as lone pairs.
Step-by-step Electron Count:
- Oxygen brings 6 valence electrons.
- The added proton (H⁺) contributes no electrons, just a positive charge.
- Three electrons are used to form three sigma bonds with H atoms (one from O and one from each H).
- This leaves one lone pair on oxygen.
So, there are:
- Three bonding pairs (O-H bonds),
- One lone pair on the oxygen atom.
This gives a total of four regions of electron density around the central oxygen atom. According to VSEPR theory, four regions of electron density arrange themselves in a tetrahedral electron geometry. However, the molecular geometry (which considers only the positions of atoms, not lone pairs) is based on the arrangement of the three hydrogen atoms and the lone pair.
Because of the one lone pair, the actual molecular geometry is trigonal pyramidal. This is the same shape as ammonia (NH₃), which also has three bonds and one lone pair.
Summary:
- Electron geometry: Tetrahedral
- Molecular geometry: Trigonal pyramidal
- Bond angles: Slightly less than 109.5°, due to lone pair repulsion.
