differing electronegativity
the electronegativity is a chemical property that describes the tendency of an atom to attract a shared pair of electrons towards itself.
in ionic bond one of the 2 atoms attracts the electrons from another and doesn’t share it, so the electronegativity must be different to make the attract tendency different.
so,
first, when we speak about bond we should deal with electronegativity, not the number of the neutrons or the melting points
Second, if the two electronegativities are similar, then the electrons will be shared as in covalent bonds. But if they are different, one will lose the electrons to another one.
