My book says,
When NaCl forms, what really happens is that $\ce{Na+}$ and $\ce{Cl-}$ come together and form an ionic bond and thus NaCl is formed and it is a quite exothermic reaction.
Why and from where is this energy coming from? Ok Ok, I read that every system wants to reduce energy, but where literally is this energy coming from, is it because of some electrons or from the nucleus? Or from where?
The negative standard enthalpy of formation of sodium chloride (i.e. ‘the formation is exothermic’) is primarily due to the formation of a crystal lattice. This crystal lattice is a macroscopic arrangement of ions in a cubic fashion with each sodium ion surrounded by six chlorides and each chloride ion surrounded by six sodium ions.
If it were not for the generation of this lattice and its inherent stabilising properties (a very good charge distribution, charges neatly alternate, contact between species of identical charge is strongly inhibited) the formation of sodium chloride would be much less exothermic.
Note most importantly, that it is not merely the generation of sodium cations (that process is endothermic) or the generation of chloride anions (that process is only weakly exothermic) that causes the overall exothermicity of the entire reaction.
The answer is lattice energy.
The enthalpies involved in the formation of ionic compounds can be understood by Born-Haber cycle.
The following steps are involved in the formation of $\ce{NaCl}$ for example.
$$\ce {Na(s) -> Na(g)}\tag{1}$$
$$\ce{Na(g) -> Na+ + e-}\tag{2}$$
$$\ce{1/2 Cl2(g) -> Cl(g)}\tag{3}$$
$$\ce{Cl(g) + e- -> Cl^{-}(g)}\tag{4}$$
$$\ce{Na+(g) +Cl^{-}(g) -> NaCl(s)}\tag{5}$$
You can see that energy is required in the steps 1-3 whereas energy is evolved in 4th and 5th steps.
The energy evolved in the 5th step is called the lattice energy of the ionic compound and this only is responsible for the overall process being exothermic.
Now why lattice formation is exothermic? This is because when a lattice is formed, new attractive forces are set up that are dominant over the new repulsive forces set up. As it is known that attractive forces decrease the potential energy of a system(or increase the stability), so energy is released as heat.
Why and from where is this energy coming from?..
If we go much deeper it can be explained physically by the famous equation (energy mass equivalence) $ E = M C^2 $. So the energy comes from the mass defect which gets converted to energy.( I hope you know what mass defect is). As we know that energy released in a chemical reaction is very small in comparison to a nuclear reactions, so the mass defect in chemical reactions is also very small in comparison to nuclear reaction. So in general we do not take into consideration the mass defect in a chemical reaction(since it can be neglected) So we generally explain evolution of energy on the basis of other energies but all of these point again to energy mass equivalence.
At the first step in the formation of NaCl, the solid Na should be converted to gaseous Na by the process sublimation so there comes sublimation entalpy, secondly the gaseous Na should lose one electron to become a Na+ ,so there comes the role of ionisation entalpy and if we see the case of chlorine molecule at first they need to dissociate to 2 cl and of electron affinity they should get an electron becoming stable. At last we need to bring both of them to an equilibrium position such that the released energy is sufficient for them to make a bond (lattice energy) then in the same Na+and Cl-ions the electron and the protons in the nucleus attract so some energy is again spend against that. Thus while looking overall we can under that the process of formation of NaCl is an exothermic process
