[Enthalpy]

And since the orbitals of Na and Cl are at the same place, how should you tell whether an electron belongs to one or the other? "In the neighbourhood of Cl" is also the neighbourhood of the 6 surrounding Na. And then, how to tell whether the bond is polarized?

[Corribus]

Cant you distinguish between ions and covalently bonded atoms by Xray difraction?

As fas as I know X-Ray Diffraction is used to determine the interatomic distance . Can you please elaborate your view ?? I have not read X-ray diffraction in so much detail .

I think he's thinking of X-ray photoelectron spectroscopy, which is routinely used to determine the oxidation states and chemical environments of atoms/ions. I don't see why it couldn't be used (in principle) to determine whether the atoms in NaCl are ionized or not.

https://en.wikipedia.org/wiki/X-ray_photoelectron_spectroscopy

Maybe there's not something I'm missing, though. I don't have a whole lot of experience with this technique.

[AdiDex]

And since the orbitals of Na and Cl are at the same place, how should you tell whether an electron belongs to one or the other? "In the neighbourhood of Cl" is also the neighbourhood of the 6 surrounding Na. And then, how to tell whether the bond is polarized?

May be there is some confusion , I said that when two waves interact with each other (overlap with each other) ,they looses their individuality . Same thing happes in the case of NaCl bond , when their electrons overlap with each other , they are no more bound to either Na or Cl , actually They are bound to both of them (As far as I know , I can be wrong , please correct me If I go wrong) .
Then I said my question is quite answerable if we consider the electron probability Density . Of-course It can be measured .
Initially i posted this  because my sir told me that Principle of Structure and Reactivity by James E Huheey is one of the most Reliable  book in Chemistry .

I was uncertain about the meaning of this paragraph . Is there any deep reasoning behind this ? Which I am not able to understand ??
"We have no absolute proof of the existence of ions in solid Sodium Chloride . For example though our best observation is that ions are found when it is melted or dissolved in water does not prove that they existed in the solid crystal."

[Irlanur]

I think there should be atoms , How can you define molecule without atom ?? I think here is the question about their bonding ...!!

You can easily define molecular structure with the positions of the nuclei and some electron density. But who is going to tell you were atom x ends and the next one starts?

You can even go one step further. If we don't use the Born-Oppenheimer approximation of fixed nuclei, we don't even know what we mean by "chemical structure".

[AdiDex]

we don't even know what we mean by "chemical structure".

What do you meant by this ?? I didn't understand . Chemical structure ...!! Please elaborate it .