What affects the strength of the ionic bonds? Is it just the electrostatic forces of attraction between the ions or it is the number of ions? For example, when comparing Al2O3 and Na2O,the aluminium oxide has a higher melting point. So do we say that in Al2O3, aluminium ion has a charge of 3+ and the oxygen has a charge of 2- while in Na2O, the sodium only has a 1+ charge. Hence the forces of attraction between the Al and O is stronger than compared to Na and O.
Or, do we say that there are more oppositely charged ions in Al2O3 than in Na2O which causes the Al2O3 to have a higher melting point?
I'm guessing that we compare them by their electro negativity as in the lattice, the ionic bonds are between the two ions and it's only because of their charges which causes them to have a different ratio of ions but eventually it only the interaction between the two ions affects the ionic bond's strength?