Difference between binding energy and ionization energy

[Ado]

Hi!
I wonder since some time why ionization energies are not equal to binding energies.

For instance, take the case of aluminum in the following configuration [Ne]3s²3p

Ionization energy ~ 6 eV
Binding energy ~ 5 eV

Where does this difference come from ?

Thanks in advance

 

[DrClaude]

What is your source for these numbers (especially the binding energy)?