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.

  • $\begingroup$ entropy likes chaos $\endgroup$ – Charlie Crown Mar 11 '19 at 3:54
  • $\begingroup$ There is a natural tendency for a system to come to rest at the lowest-energy state. $\endgroup$ – tschoppi Mar 11 '19 at 8:23
  • $\begingroup$ Intuitive? Chemistry depends on observation. Specific to the general, not the other way around. Remember Galileo and the Tower of Pisa! $\endgroup$ – user55119 Mar 11 '19 at 13:03

What makes some atoms/compounds reactive and some noble?

Unpaired electrons and how tightly these electrons are bound to their respective nuclei.

Why do reactions happen at all? What is the fundamental truth behind a reaction?

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Because atoms are more stable together than they are apart, and systems will tend towards their lowest energy state.

But why NaCl?

This is really a matter of ratios, which is the basis of chemistry. Different elements share their electrons in different ways under different circumstances. Some can donate their electrons ($\ce{Na}$, $\ce{Al}$, $\ce{Au}$), some accept electrons ($\ce{Cl}$, $\ce{O}$, $\ce{N}$), and others are somewhere in between ($\ce{C}$, etc.). Sodium and chlorine are very consistent with how they share electrons. Sodium has 1 valence electron, chlorine lacks 1 valence electron, and they combine in a one-to-one ratio. The formula unit of their salt is $\ce{NaCl}$.

When considering the salt of aluminum and chlorine, ask how many valence electrons does each atom have/need. I think you might begin to see the pattern.

$[1]$ Bond energy diagram

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