What is meant by 'stability'?

I'm currently learning about substitution and elimination reactions and to do so I need to learn about bases and nucleophiles. 


Apparently a good base is an **unstable** species. 


What does stability even mean in chemistry?


I just need to conceptually understand and wrap my head around this concept of a compound or atom or species being 'stable' or 'unstable'. Please don't explain it in complicated terms.




Stability has several meanings. A compound can be stable on its own, under the given conditions such as temperature and pressure. This is the case for a lot of compounds. The other definition of stable could be a compound's inability to react with other molecules (like the inability of carboxylic acids to be oxidized). This is probably what you meant. A good base, like $R-O^-$ is unstable because of the +I effect of the alkyl group, which shifts the electron density towards oxygen, which in turn makes the ion a very good and strong nucleophile. This is the reason why such "good bases" tend to cause eliminations and substitutions in organic molecules. 


P.S.: This is the simplest explanation I can provide. If you have any doubts, ask them in the comments.


P.S.S.: Stability of a compound can also be explained on the basis of what @NotNicolaou said.


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Stability is when a situation is at a relative free energy minimum OR when there is no possible or reasonable mechanism for something to happen. An historical example: nitroglycerine was a favored explosive in mining etc. Nitro, undisturbed, was stable; but if you shook it or jarred it too hard it exploded, severely damaging the person who shook it; Along comes Alfred Nobel; he figures a way to stabilize nitro and invents dynamite; dynamite sticks can be dropped They are stable! or are they? The profits started the Nobel prizes.


Most compounds are stable if in isolation including strong bases or acids; some compounds are inherently unstable, an example being Pure H2O molecules. They cannot exist, for very long at least, without self-ionizing to H3O+ and OH- ions. So the combination of a strong base mixed with a very weak acid is a mix that might be unstable ie not at equilibrium. It might react if a mechanism is possible and if the base and or acid are strong enough for a reaction.


Stability is an unfortunate word to describe reactivity. A base such as OH- is more reactive than H2O when reacting with acids. Stability depends on the conditions and displacement from equilibrium.


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