Water has four electron pair sites around the central oxygen atom. However, two
of them are lone pairs, so water’s molecular geometry is as follows:
It’s another variant of the tetrahedron, but with two corners occupied by electron
pairs. This molecular geometry is known as bent. The angle between O–H bonds
is about 105°.
The central molecule need not have four electron pair sites. If it has two, the
molecular geometry will be linear, with 180° between bonds. If it has three
electron pair sites (and no lone pairs), the sites will be 120° apart and the
molecular geometry is trigonal planar. Planar means that the molecule is flat or
two-dimensional. If there are three sites and one is a lone pair, the shape
resembles that of the water molecule and is also called bent.
You should also be aware of two elements that violate the octet rule. Beryllium
(Be) atoms are stable with 4 valence electrons. When beryllium is the central
atom, the molecule is linear. Boron (B) atoms strive to gain 6 valence electrons.