In this website, http://hyperphysics.phy-astr.gsu.edu/hbase/molecule/nacl.html , it says that it only takes 1.52 eV to create both ions. And when the two ions form come close to 0.94 nm, they have potential energy of -1.52 eV. Then is says that if the two neutral atoms come closer than 0.94 nm, it is energetically favorable to form ionic bond. But what does it mean by energetically favorable? If the two neutral atoms get closer than 0.94 nm, where do they get the energy to ionize each other? Also, why some curve tends to zero as the separation tends to infinity, whereas some tends to a certain number as the separation tends to infinity? Thank you.