Help understanding ionic compound properties.

Announcements
    • Thread Starter
    Offline

    0
    ReputationRep:
    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.
    • Thread Starter
    Offline

    0
    ReputationRep:
    Thanks for the replies guys :/
    Offline

    3
    ReputationRep:
    (Original post by mosam00)
    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!
    • Thread Starter
    Offline

    0
    ReputationRep:
    (Original post by MexicanKeith)
    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.
    Offline

    3
    ReputationRep:
    (Original post by mosam00)
    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.
    If you look here http://www.chemguide.co.uk/inorganic...xidesphys.html at the second section of green text entitled "Problems!" you can see that some explanations are offered for the aluminium vs magnesium oxide side of things, what it boils down to is the fact they don't addopt the same 3d structure so you aren't comparing like with like.

    Chlorine appears in the period below oxygen in the periodic table, this means its outer electrons are in higher energy orbitals than those on oxygen. This leads to the electron cloud around Cl- being much larger than that on O2- and hence Cl- is more easily polarised (a friend of mine at university likes to think of bigger ions as being "flabby" and their electron cloud easily distorted). This combined with Al3+'s polarising nature leads to covalent bonding being dominant (although the bonds are polar so some ionic attraction still occurs).

    The fact this covalent compound has a much lower melting point is because, in an ionic compound you must break the ionic interactions to melt, but in a covalent compound the covalent bonds remain in tact and only the intermolecular forces between AlCl3 molecules must be broken. This is true of many ionic and covalent compounds.
    Offline

    2
    (Original post by mosam00)
    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.
    (Original post by mosam00)
    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.
    At A level you won't get a satisfactory answer to this but both are true to an extent; as you go from eg +1 charge to +2 in the cation you will get an increase in the melting point of an ionic compound, the reason being a stronger electrostatic attraction between the ions.
    Some ions with a high charge density (lots of charge, small volume,eg Al3+) will be polarisaring so that it takes electron density from the anion which results in more covalent character. However this will depend on the identity of the anion,as Keith said, because the anion itself needs to be polarisable (usually larger so electron density more weakly attracted to the nucleus) eg S2- would be but O2- isn't. The first theory is true up until the charge gets high enough that the compound is more covalent in character- which is why they aren't contradictory. Neither Na+ nor Mg2+ are sufficiently polarisaring to ever have much covalent character so you'd expect Mg ionic compound to have higher melting points than Na for the same anion.
    With Al it gets more complicated as said by Keith, I doubt you'd get data for this kind of question for this reason :P

    Aluminium chloride is less ionic than Aluminium oxide because Cl- is polarisable, the Cl anion will be quite a bit larger because it's on the 3rd row vs 2nd row. Therefore more covalent character in AlCl3 hence lower MP. I don't think electronegativity has anything to do with it, usually it's only discussed for atoms not ions

    Hope that's helped (and to everyone else, if I've got anything wrong please correct me!)
    • Thread Starter
    Offline

    0
    ReputationRep:
    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
    Offline

    3
    ReputationRep:
    (Original post by mosam00)
    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.
    • Thread Starter
    Offline

    0
    ReputationRep:
    (Original post by MexicanKeith)
    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.
    Ok thanks
 
 
 
Write a reply… Reply
Submit reply

Register

Thanks for posting! You just need to create an account in order to submit the post
  1. this can't be left blank
    that username has been taken, please choose another Forgotten your password?
  2. this can't be left blank
    this email is already registered. Forgotten your password?
  3. this can't be left blank

    6 characters or longer with both numbers and letters is safer

  4. this can't be left empty
    your full birthday is required
  1. Oops, you need to agree to our Ts&Cs to register
  2. Slide to join now Processing…

Updated: September 20, 2016
TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

Poll
Do you think sex education should be compulsory in schools?
Study resources

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

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