[a confused chiral girl]

Hi,

I drew the structure of boron triflouride (BF3) with the F atom in the center, connected to 3 floride ions as the Lewis structure. Is this the correct lewis structure for this molecule? I am confused as to know if/when there should be double bond?
thanks!

[refid]

not really 100% sure about the explanation, but boron has a electron gap (sp2) thats why it bound to 3 atoms.. again im not sure

[english]

Boron's funny.  It's planar, according to VB theory....physical chemists will tell you otherwise. 

Anyway, boron is said to be a natural electron-deficient element.  That is to say, it always has an empty p orbital in its structure.


So basically, B has no lone pairs, and is just triply bonded to the three fluorides.  There's chemists that devote their whole careers just on boron.  Boron chemistry....ugh.   

[a confused chiral girl]

whole career on boron..

but BF3 has resonance structures, and some of them have double bonds. when can we tell in what cases or under what circumstances, that we need to draw in double bonds?

[english]

Well to get a resonance structure for boron trifluoride you must fill the unoccupied p orbital of boron by moving a lone pair of one of the fluorines as a pi bond.

Basically, the image below.  This gives boron a negative charge and fluorine a positive charge.  Not only do you introduce separated charges (bad), but you have given the most electronegative element a positive charge (ugh).  

There is delocalization in BF₃, but no resonance.  Resonance implies that your resonance structures are relatively stable.

[a confused chiral girl]

Well to get a resonance structure for boron trifluoride you must fill the unoccupied p orbital of boron by moving a lone pair of one of the fluorines as a pi bond.

There is delocalization in BF₃, but no resonance.  Resonance implies that your resonance structures are relatively stable.

So..are you saying that the picture you drew is a resonance structure of boron trifloruide, but ..BF3 does not have any resonance? 

[refid]

whole career on boron..

but BF3 has resonance structures, and some of them have double bonds. when can we tell in what cases or under what circumstances, that we need to draw in double bonds?

Are you asking about exceptions to the octet rule?

[english]

Well to get a resonance structure for boron trifluoride you must fill the unoccupied p orbital of boron by moving a lone pair of one of the fluorines as a pi bond.

There is delocalization in BF₃, but no resonance.  Resonance implies that your resonance structures are relatively stable.

So..are you saying that the picture you drew is a resonance structure of boron trifloruide, but ..BF3 does not have any resonance? 

Yep.   

Tricky concept. When we say "resonance structures," they are merely hypothetical.  If they aren't very stable, then there is no resonance.  There is only delocalization.  The two terms are quite different.    

[a confused chiral girl]

whole career on boron..

but BF3 has resonance structures, and some of them have double bonds. when can we tell in what cases or under what circumstances, that we need to draw in double bonds?

Are you asking about exceptions to the octet rule?

I know boron only has 6 electrons in its orbit. but there are also resonance structures with double bonds between the C=F, which I wasn't aware of. so I am wondinerg how do we know when there are double bond in the resonance structures or when we have to draw delocalization structures

thanks

[english]

The resonance structures of BF₃ are so unstable that you mustn't draw them.  Under normal circumstances, when the contributors are relatively stable, you draw them all.  All of these contributors would be that particular compound. 

A compound is not one contributor one minute and another the next.  It's all of them at once.  We just can't depict this.  Usually, dashed bonds indicate this reality.


BF₃, however, does not really have any resonance contributors.  You can draw these contributors and see how unstable they are, and therefore ignore them.  This implies that BF₃ follows the general structure indicated by VB theory. 

A very general explanation of VB theory.