Question

Question #73746

Answer 1

Actually it doesn't. In the planar molecule the `2s` orbital on oxygen mixed with only the two `2p` orbitals in the same plane to make the bonds.

Explanation:

A diagram of the molecular orbitals is given here. Note that the `2p_z` remains unchanged.

https://en.wikipedia.org/wiki/Molecular_orbital_diagram

Answer 2

It is a consequence of both Hund's rule of maximum multiplicity (which gives oxygen two unpaired electrons) and achievement of maximum separation of the orbitals.

Explanation:

Following Hunds rule electronic configuration of oxygen is `1s^2 2s^2 2px^2 2py^1 2pz^1`.

Oxygen, therefore, has two unpaired electrons which can bond with the two hydrogen atoms. In order to obtain maximum separation of the electron orbitals from the 3 atoms, bonding is achieved by the formation of four `sp^3` hybrid orbitals. However, two of these contain only non-bonded pairs, so the overall shape looks more like a "bent" V than a normal tetrahedron. Repulsion from the remaining lone pair on the oxygen atom means the bond angle is reduced to around 105 degrees..