Turn on thread page Beta

Why does Aluminium Oxide have a lower melting point than Magnesium Oxide? watch

    • Thread Starter
    Offline

    12
    ReputationRep:
    Hi,

    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.
    Offline

    11
    ReputationRep:
    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.
    • Community Assistant
    • Study Helper
    Offline

    17
    ReputationRep:
    Community Assistant
    Study Helper
    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.
 
 
 
Reply
Submit reply
Turn on thread page Beta
Updated: March 2, 2018

University open days

  1. University of Bradford
    University-wide Postgraduate
    Wed, 25 Jul '18
  2. University of Buckingham
    Psychology Taster Tutorial Undergraduate
    Wed, 25 Jul '18
  3. Bournemouth University
    Clearing Campus Visit Undergraduate
    Wed, 1 Aug '18
Poll
How are you feeling in the run-up to Results Day 2018?

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

Write a reply...
Reply
Hide
Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.