Potential energy curve of Sodium Chloride

[slft]

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.

[slft]

Push~~

[DrDu]

First I want to point out that the ionization does not occur suddenly when the distance of 0.94 is reached. It is a gradual process where the bond changes from being of mainly covalent character at larger distances to higher ionic character at lower distances.
At infinite distance, the energy of the free atoms is taken as the origin of the energy axis. At infinite distance, the energy of two ionized atoms is higher than that of two neutral atoms, which is not astonishing. When the distance is lowered, the Coulomb attraction between the two charges lowers the energy of the two ions relative to the energy of the neutral atoms (which at small distances also interact to form a covalent bond).