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Homework Help: Ionic vs. Covalent

  1. Nov 3, 2009 #1
    How do you know whether AsI3 is ionic or covalent?

    I was asked on an exam to name it, and I called it Arsenic triioide when the prof said it was Arsenic (III) iodide.

    I asked a sophomore TA one time how to tell when it's covalent, and she said the beginning column of the p subshell elements (the end of the transition metals) is her dividing line. In other words, if they're both on the left right of that line, it's a covalent compound, and if they're on opposite sides it's ionic.
    But apparently this is an exception, so she must have been mistaken.

    And according to another exam written by students in the chem club, Sb2Te3 is ionic, so the dividing line isn't the metal-nonmetal dividing line.
    It's like you have to know the electronegativity differences or something, which they never give you.

  2. jcsd
  3. Nov 4, 2009 #2

    You are totally right at your point and this is what I know exactly, as there is a kind of difference between these two things.

  4. Nov 4, 2009 #3
    So, you're saying the cation/anion dividing line for ionic compounds is between the pink and yellow (below), NOT the red line?

  5. Nov 5, 2009 #4


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  6. Nov 23, 2009 #5
    thank you so much for the link.
    I find it kind of overwhelming, though. Is there any way you can answer my question directly? I just want to know whether you would use the ionic or covalent rules when naming AsI3, and also Sb2Te3.
  7. Nov 24, 2009 #6


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    Honestly - I have no idea, I just know where too look for an answer when I need one.

  8. Nov 24, 2009 #7


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    As far as I can remember, IUPAC nomenclature makes no difference in nomenclature between a covalent and an ionic substance.
  9. Nov 24, 2009 #8


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    http://www.iupac.org/publications/books/rbook/Red_Book_2005.pdf [Broken]
    contains the newest recommendations on inorganic nomenclature. Although the specification via oxidation state is still allowed, it is often not the preferred choice:
    "It is possible to provide information on the proportions of the constituents in names by using one of two other devices: the charge number, which designates ionic charge, and the oxidation number, which designates oxidation state. In nomenclature, the use of the charge number is preferred as the determination of the oxidation number is sometimes ambiguous and subjective. It is advisable to use oxidation numbers only when there is no uncertainty about their assignment."

    Btw: Could it be that your prof enjoys nit-picking?

    ed:I just saw that Borek already specified that link
    Last edited by a moderator: May 4, 2017
  10. Nov 24, 2009 #9
    Looking at electronegativity differences, from this table:
    AsI3 and Sb2Te3 are certainly covalent, especially the second.
  11. Nov 28, 2009 #10
    I'm a first year college chem student and please consider that with my answer, which is:

    An Ionic bond is between a "+" and a "-" substance.
    As is "-3" and I "-1" so they would have to be covalently bonded.

    Another "rule" is ionic bonds are between metals and non metals. Sb2Te3 ;2 metals=covalent

    Most of the common ionic compounds we deal with are those between Group 1A or 2A bonding with Groups 5A, 6A, 7A.


  12. Nov 30, 2009 #11
    2 metals do NOT make a covalent bond; they make a metallic bond.
    Covalent bonds are between two non-metal elements, and ionic bonds are between non-metal and metal elements. If you delve further, you will also find out that noble gases make ionic compounds.

    Arsenic(III) Iodide would be a covalent bond, as both are NON-METALS. Remember, Arsenic is a metalloid; it's a non-metal, but with metal properties. Iodine is also a non-metal, therefore making AsI3 a covalently-bonded substance.

    With more complex compounds, involving two metalloids(such as Sb2Te3; neither of them are metals. They are metalloids, do not get confused Whalstib) you will have to know about electronegativites of different elements. For Sb2Te3, we would have to subtract the electronegativities from each other. Typically on a scale of 0 - 3.3 (0 being completely covalent, 3.3 being completely ionic) you will be looking for differences of greater than around 1.7-1.9 for more Ionic bonds and less than that for more covalent bonds.

    So for Sb2Te3 we take 2.1 - 1.9 which equals about .2.

    A covalent bond.

    You may have to use this for AsI3 too, to be sure, although it certainly is covalent.

    Whalstib: your answers were correct, but with the wrong principles.

    Last edited: Nov 30, 2009
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