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Bonding

Discussion in 'DAT Discussions' started by osimsDDS, Jun 18, 2008.

  1. osimsDDS

    5+ Year Member

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    I really dont understand this at all...

    Which of the following compounds are held together by ionic bonding?

    NaH, MgH2, or NH3

    I dont think any of these are but the answer is NaH and MgH2....the difference in electronegativity is so small that there is no way these can both be ionic??? or is this an exception...please explain thanks alot !!!
     
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  3. AndyK

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    NaH is definately ionically bonded. It's a sodium cation bound to a hydride anion... that is a hydrogen with two electrons. It's an extremely reactive base... google it :)

    MgH2 is the same thing except it ionizes to Mg2+ and two hydrides.

    NH3 is just ammonia... obviously covalently bonded.

    Whenever you see an alkali or alkali earth metal in a compound, most likely it is an ionic bond.
     
  4. osimsDDS

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    Hey thanks that helped, let me ask you this....

    is it because H has a negative oxidation number because it is bound to an element that has an oxidation of +1 or +2??? i think this is one of those exceptions right?? thanks
     
  5. AndyK

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    I'm not sure how to answer the question in terms of oxidation numbers. It's because it's the only way that compound can exist.

    There's no way sodium can be covalently bonded to hydrogen because of the octet rule (The sodium in "Na:H" needs 6 more electrons to fill the shell). Therefore, the only other option is to ionically bond. The sodium gives up its single valence electron to the hydrogen and as a result it becomes a cation whose octet is satisfied. Hydrogen gains an electron from sodium and becomes a hydride anion, and its "octet" is also satisfied. Then the two bond ionically due to their opposing charges.

    Hope that helped
     
  6. osimsDDS

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    nvm i just found it, yea it has to do with lower electronegativity with other elements that Hydrogen becomes an anion rather than a cation...

    Also it is good to note that the convention of writing out formulas is that cations are always before anions....example NaH vs HCl

    NaH...the Na is a cation and the H- is an anion....cations are first
    HCl...the H+ is the cation and the Cl- is the anion

    Damn the basics are starting to make sense which means everything else makes sense...
     
  7. Dmitry Malayev

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    I just simple remember it like this:
    Ionic Bonds can only be formed between a Metal & Non-Metal.

    Yes when compared to Covalently bonded molecules one can be more ionic then the other. But for the most part if they are asking which has an ionic bond, it will be a Mental bonded with a Non-Metal.
     

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