Help understanding ionic compound properties.

Thanks for the replies guys :/

Firstly, if you're worrying about ionic solids, you should consider melting points, not boiling points! Having looked those up they show the same trend

Na₂O < Al₂O₃ < MgO

When it comes to explaining melting points of these solids which are all mainly ionic in nature you need to consider how ions are attracted to one another (when they have opposite charge).We can use coulombs law if you know it, if not its simple to explain:

The energy of the interaction is proportional to the charges on each ion, this explains why Na₂O has the lowest melting point, it has low charge on the metal.

The ordering of Al₂O₃ and MgO can't be explained in this way. However it is important to note that these two compounds have quite different 3D structures. The attraction between ions also varies with the distance between them in the crystal structure.

Calculations of the energy of ionic lattices is more of a degree level problem (use of the Kapustinskii equation is usually involved) and can only really give approximate values.

Polarising ions like Al³⁺ certainly do lead to some covalency but in the case of oxides the ionic attractions dominate.

So, overall don't worry, the two theories you've been taught both have an effect and explaining melting points is very much about the balance of several factors.

At your sort of level the main thing to know is that ionic compounds have high melting points because they have an extended 3D lattice with lots of strong electrostatic attractions between the ions, the exact ordering of these melting points is less important!

Ok so why exactly does aluminium oxide have a lower melting point than Magnesium oxide, is it because it polarises the oxide ion more, but then that contradicts the idea that the ionic bond should be stronger because an aluminium ion has a greater charge on it than a magnesium ion. Secondly why is aluminium oxide more ionic than aluminium chloride, it's melting point is much higher? I would've thought that chloride ions have a greater electronegativity than oxide ions and so the chloride ions would be polarised less than the oxide ions? Yet the bond between aluminium and chloride ions is covalent, while the bond between aluminium and oxide ions is ionic, i would've though it would've been the other way round? I do know that the chloride ions are slightly bigger than oxide ions, but does it really make that much of a difference?

I'm sorry if my questions are stupid, but it is really bugging me that I don't seem to be able to understand this.

Thanks.

Ok so why exactly does aluminium oxide have a lower melting point than Magnesium oxide, is it because it polarises the oxide ion more, but then that contradicts the idea that the ionic bond should be stronger because an aluminium ion has a greater charge on it than a magnesium ion. Secondly why is aluminium oxide more ionic than aluminium chloride, it's melting point is much higher? I would've thought that chloride ions have a greater electronegativity than oxide ions and so the chloride ions would be polarised less than the oxide ions? Yet the bond between aluminium and chloride ions is covalent, while the bond between aluminium and oxide ions is ionic, i would've though it would've been the other way round? I do know that the chloride ions are slightly bigger than oxide ions, but does it really make that much of a difference?

I'm sorry if my questions are stupid, but it is really bugging me that I don't seem to be able to understand this.

Thanks.

In class we learned how some positive ions can polarise negative ions because of a high charge in the nucleus. E.g. an aluminium ion polarises an oxide ion more than a sodium ion since it is smaller and has a higher positive charge and there is a smaller difference in electronegativity between it and the oxide ions compared to the sodium ions. This makes the bond between aluminium and oxide ions less ionic than the bond between sodium an oxide ions. This means aluminium oxide acts less as an ionic compound and more like a covalent compound than sodium oxide and so should have a lower melting/boiling point. Which I understand. But then I learned that ionic compounds which have smaller ions with a greater charge e.g. an aluminium ion compared to a sodium ion have stronger ionic bonds and so should have higher melting points. Which I also understand. But these appear to completely contradict each other. There is something about charge density which I'm not sure I completely understand but still. Then I searched up the boiling points of Sodium oxide, magnesium oxide and aluminium oxide. Aluminium oxide has a higher boiling point than sodium oxide but less than magnesium oxide, here neither of the theories seem to work???? I'm extremely confused, pls help.

Ok so I understand that Magnesium Oxide has a higher melting point than Aluminium Oxide because the bonds in Aluminium Oxide are more covalent in character since Aluminium ions are more polarising, but why then is it's melting point still higher than sodium oxide, if its bond is more covalent? @MexicanKeith @Poooky

None of these differences can be boiled down to simply saying "more/less covalent/ionic" the differences in three dimensional structure, the ionic radii, the ionic charge, all have effects as well as other things. Properly understanding all these effects and their influence on melting points is not something that can be rationalised in the space of a few sentences or paragraphs.