Why is ozone, ##O_3##, bent and not linear ?

The of the ozone molecule is bent because of the lone pair of electrons present on the central oxygen atom.

There are at least three used to describe the ozone molecule, but I'll only focus on two of them, the major contributors to the actual hybrid structure.

Notice that the central oxygen atom is bonded to two other oxygen atoms and that it has a lone pair of electrons present.

That lone pair of electrons takes up more space than the bonding electrons. As a result, the repulsion that exists between the bonding electrons and the lone pair will push down on the bonding electrons and force the molecule to adopt a bent .

This is exactly what happens in the water molecule, the only difference being that water has two lone pairs of electrons that surround the central oxygen atom.

That is why the ozone molecule has a larger bond angle, i.e. it's not as bent, compared with the water molecule.