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AQA A2 CHEM5 Ionic bonding with covalent character

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    Hey could anybody please help me? in general i get the idea of ionic bonding with added covalent character, that a small positivly charged ion can distort the electron cloud of large negative ion so there is some degree of electron sharing.

    However i was wondering does Ionic bonding with added covalent character such as in NaCl or MgCl2 make the bonds slightly stronger or slightly weaker compared to the pure ionic model?

    I assume it makes them stronger since the lattice formation enthalpy is more negative compared to the pure ionic model and therefore the lattice dissociation enthalpy is more positive meaning more energy is required to break the bond. BUT this does not explain why Al203 has a smaller melting point than Na2O or MgO which are purely ionic? I am really confused.

    Also why are large positive ions such as Cs+ bonded with small negative ions such as F- almost purely ionic? does covalent character only occur with small positive ions with large negative ions?

    Any help is much appreciated, thank you
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    Oh wait i suppose NaCl and MgCl2 are not purely ionic but have some covalent character, i think I am kind of confusing myself!
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    (Original post by raveen789)
    Hey could anybody please help me? in general i get the idea of ionic bonding with added covalent character, that a small positivly charged ion can distort the electron cloud of large negative ion so there is some degree of electron sharing.

    However i was wondering does Ionic bonding with added covalent character such as in NaCl or MgCl2 make the bonds slightly stronger or slightly weaker compared to the pure ionic model?

    I assume it makes them stronger since the lattice formation enthalpy is more negative compared to the pure ionic model and therefore the lattice dissociation enthalpy is more positive meaning more energy is required to break the bond. BUT this does not explain why Al203 has a smaller melting point than Na2O or MgO which are purely ionic? I am really confused.

    Also why are large positive ions such as Cs+ bonded with small negative ions such as F- almost purely ionic? does covalent character only occur with small positive ions with large negative ions?

    Any help is much appreciated, thank you
    Cs+ can't approach the F- ions as closely due to its size therefore it can't distort the electron cloud as much. The small F- ions don't have as many electrons therefore it has a far smaller electron cloud. This means the polarisation of the CsF ion is negligible therefore there's very little covalent character.

    You get increased polarisation with:
    • Large, highly charged negative ions
    • Small, highly charged positive ions


    For reasons similar to the above - a small highly charged positive ion can approach the negative ion closer so it can distort the electron cloud more strongly. A large highly charged negative ion has a greater electron cloud that can be distorted.
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    (Original post by raveen789)
    Hey could anybody please help me? in general i get the idea of ionic bonding with added covalent character, that a small positivly charged ion can distort the electron cloud of large negative ion so there is some degree of electron sharing.

    However i was wondering does Ionic bonding with added covalent character such as in NaCl or MgCl2 make the bonds slightly stronger or slightly weaker compared to the pure ionic model?

    I assume it makes them stronger since the lattice formation enthalpy is more negative compared to the pure ionic model and therefore the lattice dissociation enthalpy is more positive meaning more energy is required to break the bond. BUT this does not explain why Al203 has a smaller melting point than Na2O or MgO which are purely ionic? I am really confused.

    Also why are large positive ions such as Cs+ bonded with small negative ions such as F- almost purely ionic? does covalent character only occur with small positive ions with large negative ions?

    Any help is much appreciated, thank you
    It's all to do with polarisation / ability to be polarised.

    If you look at the chlorides of Period 3 elements: Na2O, MgO, Al2O3, SiO2, P4O10, SO3, Cl2O7, you will see that they become covalent (Cl2O7) from ionic (Na2O).

    If you look at the cations that would be formed in each of the compounds: Na+, Mg2+, Al3+, Si4+, P5+, S6+, Cl7+, the ions become smaller, with an increasing charge. Hence, their charge density increases. This means that they become more polarising.

    The oxide ion (O2-) remains the same in each case. Hence, as you go from left to right across Period 3, the cations become more polarising, and thus more able to distort the electron cloud of the O2- ions. Hence, there is an increase in the covalency of the bonds as you go from left to right.

    Overall

    Polarising:
    • Cation
    • Small ion
    • Large charge


    Polarisable:
    • Anion
    • Large ion
    • Small charge


    That is why CsF is purely ionic, whereas ClF5, for example, is purely covalent.

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Updated: June 4, 2012
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