@Mithoron *I you'd have unbound Na+ and Cl- and they attracted each other, they would collide and get bound * . Wait. When they will be attracted whole of the Potential energy will be converted into Na+ and Cl+'s K.E. Now when they will collide they will for sure get separated again because the energy needed for that is initial P.E. which can be given by the K.E. But this may not happen if the collision is inelastic.

@user103816 Well, if creation of new molecule happens then of course it's inelastic en.wikipedia.org/wiki/Inelastic_collision - even in normal gas collisions are often inelastic