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Thread: Effects of electronegativity on the bonding on an ionic compound? AgBr

  1. #1 Effects of electronegativity on the bonding on an ionic compound? AgBr 
    evo
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    Hi guys,

    I was wondering if someone could please help me understand this concept.

    I have always understood electronegativity as the ability of an atom in a molecule to attract a pair of shared electrons towards itself. So this would usually have to be a covalent bond, as electrons would need to shared.
    I understand that differences in electronegativity can cause polarization in a covalent molecule, forming a dipole due to the uneven distribution of the shared electrons within the bond. (BTW would this be known as a polarised covalent bond?)

    But what impact could electronegativity have on an ionic compound? After all, ionic bonding involves electrons being transferred from a cation to an anion (and does not involve sharing).

    Could someone please explain to me how electronegativity could effect the bonding within an ionic compound and what effect it would have on a compound such as AgBr?

    Many thanks

    Evo


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    Bullshit Intolerant PhDemon's Avatar
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    Electronegativity can be thought of as a tendency for an atom to attract electrons, more electronegative elements are more likely to form stable anions whereas electropositive atoms tend to lose electrons to form cations, these tendencies in combination lead to formation of ions when electronegative and strongly electropositive atoms react. Covalent bonds are formed when two atoms that are not so different in electronegativity bond, they both have a tendency to attract electrons so ions are not formed but the electrons in the bond are on average more likely to be found closer to the more electronegative atom and so the bond is polarised.


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    evo
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    Hi thanks for the reply.

    Just so I understand this correctly; Ag and Br have a difference of 0.9 in pauling scale (if I calculated correctly). Under most guides I've read, anything < 1.4 is considered covalent. However AgBr is clearly ionic? although it is highly insoluble.
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    Bullshit Intolerant PhDemon's Avatar
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    You can't really talk in absolutes like that. There are as many exceptions to these generic rules as there are compounds that adhere to them. There is always a grey area in these sort of concepts. AgBr is ionic but there is some considerable covalent character. These simple rules of thumb like the one you quoted are designed to teach the simple general principles, as you study chemistry you will see it is actually a lot more complicated than these rules imply and all sorts of things don't fit with the simple rules you learned in high school. This is when the subject becomes interesting
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    KJW
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    If one compares the fluoride ion with the bromide ion, then the fluoride ion is much smaller and therefore the negative charge is distributed over a smaller volume. Positive charges and the positive ends of dipoles can get closer to the full negative charge and the resulting electrostatic stability will be higher.
    There are no paradoxes in relativity, just people's misunderstandings of it.
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    evo
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    Quote Originally Posted by KJW View Post
    If one compares the fluoride ion with the bromide ion, then the fluoride ion is much smaller and therefore the negative charge is distributed over a smaller volume. Positive charges and the positive ends of dipoles can get closer to the full negative charge and the resulting electrostatic stability will be higher.
    So would that mean the bonding between the atoms of an ionic compound would the stronger?
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  8. #7  
    KJW
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    Quote Originally Posted by evo View Post
    Quote Originally Posted by KJW View Post
    If one compares the fluoride ion with the bromide ion, then the fluoride ion is much smaller and therefore the negative charge is distributed over a smaller volume. Positive charges and the positive ends of dipoles can get closer to the full negative charge and the resulting electrostatic stability will be higher.
    So would that mean the bonding between the atoms of an ionic compound would the stronger?
    Yes. It becomes even stronger if the positive ion is small as well. But note that this is only part of the story as far as solubility is concerned. The amount of energy required to separate the ions of a crystal can be quite easily supplied by the interaction of the ions with the solvent molecules. Chemistry in general is often about the balancing of opposing effects, sometimes quite large effects.
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    There are no paradoxes in relativity, just people's misunderstandings of it.
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    KJW
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    I should remark that when it comes to crystals, there is also the way the ions pack together to form a regular structure to consider. Not only is there the attraction between opposite charges but also the repulsion between like charges as well. Also, the relative sizes of the ions is an important consideration since ions that are similar in size geometrically pack better than dissimilar sized ions.
    There are no paradoxes in relativity, just people's misunderstandings of it.
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    evo
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    Great I think I've understood that. Thanks guys
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