# Why is the electronic geometry of water molecule tetrahedral, but we describe its geometry as "bent"?

Aug 10, 2017

I think you are almost there......but you could be more precise.....

#### Explanation:

And I acknowledge that English may not be your first language (in which case you are doing well!). In simple $\text{VSEPR}$ the geometry of electron pairs, however many there are, are determined by the number of electron pairs. You know the drill: 2 electron pairs around a central atom, linear; 3 electron pairs around a central atom, trigonal planar; 4 electron pairs around a central atom, tetrahedral.

But we determine molecular geometry on the basis of the disposition of ATOMS not on the basis of the disposition of electron pairs. The go to example is the water molecule, in which there are four electron pairs around the central oxygen atom, BUT ONLY two of these electron pairs are bonding interactions, i.e. that represent $O - H$ bonds.

The geometry of electron pairs in WATER is thus tetrahedral to a first approx....but we describe molecular geometry on the basis of bound atoms not on the geometry of electron pairs. Water is thus a bent molecule with $\angle H - O - H \cong {105}^{\circ}$ because the lone pairs, which lie close to the oxygen atom, tend to constrain the $\angle H - O - H$ angle down from the ideal tetrahedral geometry of ${109.5}^{\circ}$.