Why do reactions happen at all? [closed]

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.

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

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