Why is NaCl in the gas state covalent?

[Raziel2701]

I'm supposing that it has something to do with the fact that it is no longer in a lattice, so it's just one Na and one Cl ion bonding together but why would it be a mostly covalent bond in this case?

 

[JohnRC]

It is all a matter of energy. The bond may be regarded as very polar covalent or ionic, for whatever purpose you might want. Nature does not know about chemical bonds!

However, if you try to dissociate gaseous NaCl, the lowest energy pathway is to
Na^• + Cl^•
rather than
Na⁺ + Cl^–
Because the energy cost of removing the electron from sodium (ionization potential) is larger than the energy return from the chlorine atom picking up the extra electron (electron affinity).

That is the real reason why for most, but not all, purposes, the bond in gaseous NaCl should be regarded as polar covalent.

 

[DrDu]

Due to the long range of the Coulomb interaction, bond dissociation in vacuo always leads to neutral species.
What is maybe interesting is to look at the potential energy surfaces of NaCl in the gas phase.
http://t2.gstatic.com/images?q=tbn:ANd9GcSJPBDQhxnHyh-6ffcruCX2uZvPMS75MLhgm9oxVPlnh5-WQZ3_

This shows that at bonding distances, ionic bonding is preferred.
This is nicely discussed in Paulings book "The nature of the chemical bond"

 

[lightarrow]

I'm supposing that it has something to do with the fact that it is no longer in a lattice, so it's just one Na and one Cl ion bonding together but why would it be a mostly covalent bond in this case?

The ion form is energetically stabilized in the lattice because every single ion is surrounded by several ions of opposite charge (not one ion only).