[Polleke]

hallo all,


correct me if I am wrong, but the VSEPR theory is only used for covalent boundings right?

and another question: an ionbounding is always a bouding between a non-metal and metal right?
===>
I just want to clarify the following rule that states that when the difference between EN values is bigger then 1.7 you have a ionbounding.. however I have read somewhere that BF₃ is a concidered as a covalent bounding (they use the VSEPR theory on it). Why?

and how should I see B in the example I know its a semi metal , so I should see it as a metal (instead of a semi metal) and F a non metal.

thanks

[Astrokel]

Yes VSEPR is only used for covalent bondings and that for ionic bonding, the shape is determined by crystal lattice packing theory.

You are right that BF₃ is a covalent compound because of the very small size of boron compare to relatively large size of flourine. Imagine if they are packed in a crystal lattice, the relatively larger size of flourine will definitely cause a repelsion with each other, so the packing is not efficient at all. Therefore they would rather exist as molecule through covalent bonding.

As you might aware that there is hardly fully ionic or fully covalent, so there are things in between such as BF₃ which is a polar covalent bond, as the covalent bond is polarize by the large difference in electronegativity.

[Polleke]

ok Astrokel  ,

that is what I was thinking.
But in my papers they only wrote a very general idea on what VSPER and covalent bounding etc.. are... because of those general rules some things seemed "not" normal. So I was wondering where I got things wrong.

Too bad they never write near the "general" rule that it is a simplification of the "real" rule.