So my book says that MgO has the lower melting point than Al2O3. It explains this by saying that Aluminium is a very small ion with a large positive charge and so can approach the larger O2- and distort its electron cloud. This means that the bond has some added covalent character.
Does this just mean that the Aluminium attracts electrons from the Oxide so this means there is some covalent character? Also, surely having additional covalent character would increase the melting point of Aluminium Oxide as it would be harder to break the bonds?
Can someone please explain the concept to me if they have the time?
Thanks! Would appreciate it.
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Why does Aluminium Oxide have a lower melting point than Magnesium Oxide? watch
- Thread Starter
Magnesium oxide and aluminium oxide are considered ionic compounds because they consist of cations and anions.
However, some cations have a stronger 'electron pulling' ability (polarising power) than others. This depends on the charge density: the larger the charge and the smaller the radius, the greater the polarising power. Al3+ has more power than Mg2+.
Some electrons from the anions might be tempted to approach the metal ion. Oxide anions, alongside with other electron-rich anions, are said to be polarisable.
This leads to a non-negligible degree of covalency (sharing of electrons). Try looking for the electron density maps of the two oxides. Aluminium oxide has a higher density between the nuclei.
Covalency makes the bond directional. In some way, this pull on the electrons of an ion distorts the bond and makes it easier to break.
It follows that Al2O3 is less stable than MgO (respectively melting at 2 072 °C and 2 852 °C). Either you misread the paragraph or the author got distracted.
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Another factor will be the packing in the lattice. MgO has the same number of positive and negative ions and can pack more efficiently maximising the electrostatic attractive forces. In aluminium oxide there are three oxide ions for every two aluminium ions. There must be some compromise in the packing.