The H2O molecule, water, is actually bent or angular, not a perfect triangle. The reason why water has this specific shape is because of the arrangement of its electrons.
Understanding the Shape of Water
The shape of a molecule is determined by the arrangement of its electrons, both those involved in bonding and those in lone pairs. Water's oxygen atom has two bonding pairs (shared with the two hydrogen atoms) and two lone pairs (not shared), so the molecule can best be explained as tetrahedral. In theory, this would mean that each of the four electron pairs has 109.5 degrees between them.
The Role of Lone Pairs
According to the reference, the lone pairs are a bit more repulsive than the bonding pairs. This greater repulsion pushes the bonding pairs closer together, leading to roughly a 2.5 degree reduction in bonding angles per lone pair. Water has two lone pairs, so the initial 109.5 degree bond angle of the tetrahedral shape is reduced. This explains why the H-O-H bond angle in water is about 104.5 degrees, not the 120 degrees that would be found in a perfect triangle, and why water is an angular (bent) shape.
Key Points About Water's Shape:
- The shape of the water molecule is not a perfect triangle; it is bent or angular.
- The arrangement of electron pairs around the central oxygen atom is based on a tetrahedral structure.
- Lone pairs of electrons exert greater repulsion than bonding pairs.
- The increased repulsion from lone pairs reduces the bond angle between the hydrogen atoms.
| Feature | Description |
|---|---|
| Molecular Shape | Bent or angular |
| Basis | Tetrahedral electron arrangement, but influenced by the effect of the two lone pairs of electrons |
| Bond Angle | Approximately 104.5 degrees, less than the expected 109.5 degrees for an initial tetrahedral shape because of lone-pair repulsion effect |
| Repulsion | Lone pairs are more repulsive than bonding pairs |
| Cause | Lone pair repulsion reducing the bond angle of the H-O-H bond |
In summary, water's bent shape is the result of the repulsive forces of the two lone pairs of electrons on the oxygen atom, which compress the bond angle between the two hydrogen atoms.
