According to the Valence Shell Electron Pair Repulsion Theory (VSEPR), the geometry of a molecule depends on the number of electron pairs (regions of electron density) on the central atom of the molecule. Electron pairs on the valence shell of the molecule tend to position themselves as far apart in space as possible to minimize repulsion between them. Hence, the orientation of these electron pairs is the ultimate determinant of the observed geometry of a molecule.
Lone pairs of electrons cause more repulsion than bond pairs of electrons on the central atom of a molecule. Hence when the central atom of a molecule contains lone pairs of electrons, the molecular geometry is usually distorted from the expected geometry on the basis of VSEPR theory.
Hence, electrons are the subatomic particles which are responsible for any change in the observed molecular geometry of a molecule.
This is because oxygen (2.8.6) requires two electrons on its valence shell to attain stable configuration (2.8.8). Hydrogen (1) on the other hand requires one electron on its valence shell to attain stable configuration (2). Therefore in a covalent bond, it requires two hydrogen and one oxygen to share electrons and achieve stable configuration.
