TRN1983

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Yes, and lewis structures are probably the easiest way to depict them. I.e. look at Cl-. Cl has 7 electrons in its unbound, radical state. However it typically picks up electrons due to its high electronegativity and electron affinity. In a lewis structure it will have 8 electrons surrounding it. So, you'll have a Cl with 8 "dots" surrounding it and a "minus" for each "extra electron" (only a single minus in Cl-).
 

JMC114

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ok i see what you're saying. i think i worded my post wrong. for example, can you draw a lewis structure for AlCl3 even though it's ionic?
 

TRN1983

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Sure. I even found a pic of it :)

alcl3_struct.gif
 
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JMC114

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ok wow now i'm really confused! kaplan is saying that if you have a compound in ionic form that everything just stays in ion form without bonds. does that make sense?? Like it would be Al3+ and 3(Cl-).

thanks for all your help :D
 

TRN1983

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ok wow now i'm really confused! kaplan is saying that if you have a compound in ionic form that everything just stays in ion form without bonds. does that make sense?? Like it would be Al3+ and 3(Cl-).

thanks for all your help :D
Oh! I see what you're saying. You're talking about ionic compounds (like a salt), right? Okay, that's a different situation. AlCl3 isn't an ionic compound. It's a covalently bound compound (and is actually a Lewis Acid). Na+Cl- is an ionic compound (salt) that is held together by electrostatic interactions (i.e. F=kqQ/r^2 in physics).

In the case of an ionic compound (salt) you would, to the best of my knowledge, keep them "separate". I.e. with NaCl (table salt) you would have Na+ (which has lost its valence electron) and Cl- with eight dots around it (which has gained a valence electron from Na).
 

JMC114

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OHH! I get it now! :)

Last question, how can you tell when a compound is an ionic compound (salt) or not? How about these, (NO3)-, (H3PO4), (AlCl3), (Na3PO4)?
 

TRN1983

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Generally salts involve cations from the first two families in the periodic table (not including hydrogen)--they don't form covalent bonds, they just lose their electrons b/c that gives them an octet. That's the biggest giveaway IMHO.
 

mako4071

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The difference in electro-negativity will determine the bond type.
A difference of 0 to .3 will have a non-polar covalent bond.
A difference of .4 to 1.7 will give a polar covalent bond.
A difference of 1.8 and greater will give an ionic bond.
It's all about the electro-negativity difference.
Hope this helps.
 

Garciarussell

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Your right "mako4071" about the differences in electronegativity being the deciding factor on whether or not the compound is going to be ionic or not. However, the folks at kaplan seem to think that AlCl3 is an ionic compound.

See:
MCAT Physical Sciences Review Notes
SKU: MM4016I
p. 47 problem #9

The question asks: "Draw the Lewis structure of each of the following"

The answer to the question listed in the book says that AlCl3 is ionic when it is not ionic. That is misleading and incorrect. The difference in electronegativity between Al and Cl is less than 1.4 so it will have a polar covalent bond.
 
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