Re: why is ozone so unstable?

     Ozone actually gives up some energy when it breaks up, which is why 
it's unstable. Why? It has to do with how the atoms of oxygen bond. In 
diatomic oxygen, which takes a lot of energy to break up, the bonding is 
like this:  O=O , the two bonds each straight to the other atom. In Ozone, 
three oxygen atoms form a triangle. Now think of the bonds as pieces of 
flexible steel, like a stiff spring. There are two on each oxygen atom, 
and they stick straight out parallel to each other. To make ozone, they 
have to be "bent" so that they match up, to form a triangle, and what they 
want to do is spring back to being straight and have two ozone molecules 
make three diatomic oxygen molecules. So even though it takes energy to 
break the bonds, in ozone there is more energy stored in the bond than it 
takes to break it. That's why it takes energy (like a lightning bolt) to 
make ozone -- you have to put energy into it to make it. Why does it stay 
together at all? Well, it's sort of like balancing an egg on one end. You 
can do it, but most likely the egg will fall over pretty quickly. So ozone 
is like the balanced egg, and diatomic oxygen is like the egg after it 
tips over, in its "lowest energy state," which is where it wants to be.

Dan Berger adds:
The situation in ozone (or any other unstable compound) is not as bad as
Chris has depicted it. It's more like an egg balanced on its end in a shallow
pit (as if you shoved the end into a little modeling clay). It will still fall 
over if bumped, but the pit helps hold it up.

Now think of the walls of the pit as the energy required to break the
existing bonds in ozone. Since you get more energy back when the oxygen
atoms in ozone form new bonds to other things, the reaction is favorable;
but because it takes energy to break the bonds and get things rolling, 
some energy input is required to get things started. This is called
"activation energy" and is dealt with at www.secondlaw.com. Activation
energy is the major chemical reason we are able to exist at all, rather than
instantly burning to carbon dioxide and assorted ashes. After all, we're
immersed in a highly reactive gas--oxygen. Free oxygen is no more "natural"
than ozone. If it weren't for green plants, there would be no free oxygen
at all.