[JoGo]

I was wondering about the "lifetime" of molecules. Do molecules "decay" after a certain amount of time? If so why? If not so, why?
Thank you!

[fledarmus]

Molecules? Or atomic nuclei? We don't usually speak of molecules decaying. They react. As for atomic nuclei, there are some isotopes which are considered "stable isotopes" because nobody has ever observed any radioactive decay in them, or because by the current understanding of radioactive decay there is no decay pathway available. You can get more information here: http://en.wikipedia.org/wiki/Stable_nuclide

If you are interested in molecules spontaneously reacting, the answer is a lot more complicated.

[Enthalpy]

Could you reformulate the question?

Some molecules decompose quickly enough that it can be observed but slowly enough to be called molecules, for instance hydrogen peroxide. Do you want to call this "decay"?

[JoGo]

Yes, that is what I mean, excuse my english. But I am curious why do molecules decompose? What becomes weak enough that they do this?

[fledarmus]

It isn't so much that something get weak that makes a molecule decompose. It is that the molecule has received enough energy (activation energy) to enable it to react to form a more stable product.

Think about a stone at the top of a high mountain. The stone has a lot of potential energy because of its height, and it could roll down the side of the mountain and release a lot of energy. But to get it to roll down the mountain, you have to apply some energy to it, to overcome its inertia. Another stone bouncing against it might make it fall, or a person, pushing on it, or a tree falling close enough to it to make the ground shake a little, depending on how precariously its perched. The stone might even be at the bottom of a ditch at the top of the mountain, and then to make it fall down the mountain you would have to add enough energy to lift it up over the edge of the ditch.

That is the way molecules are. They may have a lot of chemical potential energy tied up in their bonds, but it still requires some energy input to get them to decompose. This energy can come from collisions with other molecules, from sunlight, from a physical shock (nitroglycerine, for example, is very shock-sensitive), from a spark, or from many other sources. So it isn't a question of something becoming weak enough that the molecule can decompose - it is a question of something inputting enough energy that the molecule can react.

[ATMyller]

Actually there are theories about proton decaying. So there is a chance for even the stablest of molecules to eventually undergo a transmutation which can break up the molecule, altought the half-life of the proton would be about 10³⁶ years.