What makes some atoms/compounds reactive and some noble? What is the fundamental truth behind a reaction? It seems that two species can undergo number of combination to give a compound. Say Sodium and Chlorine, it seems intutive to have number of combinations from this pair such as $\ce{Na2Cl}$, $\ce{NaCl2}$, $\ce{Na3Cl2}$, and even do not react at all. But why $\ce{NaCl}$?
I had been carrying the concept of octet rule, which I now find to be exception in some cases like $\ce{AlCl3}$. So, what determines the products of a reaction? Or is it that all possible combination are formed but only few are major products, in this case $\ce{Na2Cl}$, $\ce{NaCl2}$ etc. are formed but $\ce{NaCl}$ is major product.
I think it has to do something with energy but cannot figure out what.