Why is Al2S3 covalent, not ionic?

[Bigbirdo]

1. Which of the following best describes the bond character for aluminum sulfide (Al2S3)?
   • A. Polar covalent
   • B. Covalent
   • C. Ionic
   • D. Hydrophobic
   • E. Hydrogen bonding

The answer is B. Al is a metal and S is a non-metal, and they seem reasonably far apart to me, so I expected the bond to be ionic. Yet, it turns out to be covalent. On the test, how should I approach this question without knowing their electronegativities? Is this a exception?Are there other exceptions like Aluminum that I need to know?

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[jwan14]

I recently reviewed that question and the only reason I can think of is that Al is sometimes (although rarely) considered a metalloid. Maybe that influences whether it's ionic or covalent. Ionic character increases the further apart they are across the periodic table so idk. I'm not familiar with any exceptions like that.

 

[emminent]

The closer the difference between electrons of an atom is over the periodic table (or the smaller difference between electronegativity) tends to be more covalent than ionic. If you were to compare a group I element against Al then group I would make an ionic. And again since Al is closer to S it'll be more covalent.

 

[Bigbirdo]

Starting from which group is considered to have close electronegativities to those of non-metals? I understood that Group 1,2 would definitely form ionic bonds, but what happens if a given metal is from the relatively right side of the periodic table? It is hard to decide whether it has a similar electronegativity with a non-metals without knowing the values.

 

[Project Pat]

Al is a metalloid... ionics are metal + non-metal

 

[Bigbirdo]

Al is a metalloid... ionics are metal + non-metal

I thought metalloids act like both metals and non-metals? Doesn't that mean they can either form ionic or covalent bond?

 

[virajpatel]

Most sources say that it is is ionic. But I am not sure, if it is covalent wouldn't it be polar too?

 

[tendram]

I think that must be a typo. Looking at it initially I was inclined to agree with the answer given but after doing a double take at the empirical formula it looks ionic. A good rule of thumb to go by is if you can't come up with a reasonable Lewis structure for a molecule it is probably ionic. You can also look at the valence electrons and see that Al has 3 valence electrons and S has 6. Exchanging electrons between the two as in an ionic bond would have two Al atoms giving up 6 valence electrons total (3 from each) and the 3 S atoms receiving 6 valence electrons total (2 each) which is consistent with an ionic interaction. Also the empirical formula for ionic compounds tend to be written in a way that represents the most basic form of the crystalline structure (i.e. NaCl, Na2S, etc...). When in doubt, look at the valence electrons and try to make sense of it that way.

 

[tendram]

Unfortunately, no. As a chemistry major you are told to throw most of what you learned in gen chem out the window so I don't remember all those fancy charts And I wasn't wholly basing my decision on just the fact that switching around the valence electrons makes sense. The melting point of Al2S3 is very high (1100 C) which is a strong indicator that it is an ionic compound as well as the fact that is forms crystal lattices which are uncommon in covalently bonded molecules. It could very well be a covalently bonded molecule but just based off the information I've found it looks ionic.

Edit: After looking into it I found both ionic and covalent references to Al2S3 so I have no idea. I think my problem is that I keep looking at it in terms of upper level chem which makes things really confusing at times (when you get into metalloids and the like bonding can get weird). I guess it's just best to assume its covalent.

 

[bishbish777]

I don't think such a question would appear on the real DAT...I haven't heard of them using questions on disputed information, LOL. But hey, I could be wrong.