Hi,

Stable can be a tricky word when it comes to ATP.

ATP is a really interesting energy source - I would describe it as stable as P-O bonds are relatively strong, and ATP can exist in your body for extended periods of time. Anything that can persist and exist for an extended period of time is relatively stable.

To reconcile this with energy production - the Pi and ADP formed by using ATP in a reaction are more stable than ATP itself, resulting in a release of energy which you can couple to a reaction that uses up energy.

In this way, I usually describe species that can exist in nature as more and less stable (rather than stable and unstable), and I reserve the word unstable for something that doesn't exist for a long time, or is going to react very quickly and uncontrollably - something like a carbocation.

Hope this helps!