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Author Topic: BF3 and octet rule  (Read 14709 times)

Offline Gregorian

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BF3 and octet rule
« on: 19/03/2009 02:53:29 »
Hello,

Why doesn't BF3 follow the octet rule? It is not in the third period!!!!
As far as I know, elements in the third period can disobey the octet rule. Is Boron a moron? 


 

Offline Chemistry4me

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BF3 and octet rule
« Reply #1 on: 19/03/2009 05:12:12 »
Rules are made to be broken. :)

Quote
A facile explanation invokes the symmetry-allowed overlap of a p orbital on the boron atom with the in-phase combination of the three similarly oriented p orbitals on fluorine atoms.

http://en.wikipedia.org/wiki/Boron_trifluoride#Structure_and_bonding
 

Offline lancenti

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BF3 and octet rule
« Reply #2 on: 19/03/2009 08:17:35 »
Violation of the Octet Rule for Period 3 and up (or is it down based on the Periodic Table) is because of the presence of d-Orbitals, and usually the valency is in excess of 8 if I remember correctly.

Boron Trifluoride and Beryllium Chloride are both violations of this octet rule. Exactly why, I do not know but I assume it has something to do with stability. >.<
 

Offline Gregorian

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BF3 and octet rule
« Reply #3 on: 20/03/2009 06:39:26 »
Thank you guys

Now how come NO3- violates the octet rule in one of its resonance forms!!?? (as in the attached picture).. and yes this form exists. It was in my Inorganic Chem book  ::)



Thanks
 

Offline Chemistry4me

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BF3 and octet rule
« Reply #4 on: 20/03/2009 06:41:59 »
Do you know about orbitals/subshells? s, p, d, f etc....?

Perhaps you can have a read of http://www.thenakedscientists.com/forum/index.php?topic=19467.msg217305#msg217305 :)
 

Offline Chemistry4me

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BF3 and octet rule
« Reply #5 on: 20/03/2009 06:44:11 »
NO3- doesn't violate the octet rule, all atoms have 8 electrons.
 

Offline Raghavendra

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BF3 and octet rule
« Reply #6 on: 20/03/2009 06:49:26 »
Ya Chemistry4me is correct that you have to read more about Atomic struture, periodic tables.. and more Hybridization and also VESPER theory
 

Offline Chemistry4me

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BF3 and octet rule
« Reply #7 on: 20/03/2009 06:52:48 »
That's VSEPR (Valence shell electron pair repulsion).
 

Offline Raghavendra

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BF3 and octet rule
« Reply #8 on: 20/03/2009 07:01:17 »
oh correct !!!!
 

Offline Gregorian

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BF3 and octet rule
« Reply #9 on: 20/03/2009 07:04:32 »
Nitrogen does not have 8 electrons.. it only has 6!! why is that?
 

Offline Chemistry4me

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BF3 and octet rule
« Reply #10 on: 20/03/2009 07:31:38 »
Oh sorry, I didn't look at that diagram very closely. [:I]

But after looking at Wikipedia and other websites I'm not sure that, that the resonance structure of your's exists.
 

Offline lancenti

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BF3 and octet rule
« Reply #11 on: 20/03/2009 09:26:43 »
The Nitrate Ion is arranged in a plane at all times as it is sp2 hybridized. In that state, it is just like Boron Trifluoride. In which case Chem4Me's first answer and mine should work as well.

 

Offline lancenti

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BF3 and octet rule
« Reply #12 on: 20/03/2009 09:44:21 »
Also, I just realized that Aluminium Chloride has a similar 'problem' of trivalency. However, we could argue that because it is in a plane and the p-Orbitals perpendicular to this plane can all overlap, there is a degree of 'delocalization'. This may also be applicable for Boron Trifluoride but less likely because Fluorine is highly electronegative.
 

Offline Raghavendra

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BF3 and octet rule
« Reply #13 on: 23/03/2009 10:57:47 »
boron and aluminum in BF3 and AlCl3 for example. Both are electron deficient (only 6 valence shell electrons)

Al in AlCl3 has a +3 oxidation state, but that doesn't make it isoelectronic with neon. The Al has no valence electrons, each Cl has 2 valence electrons for a total of six which is electronic deficient.

Many elements exceed the octet rule, especially those in the third row of the periodic table and below. S in H2SO4 and P in H3PO4 are examples like the other guy said, but those certainly aren't the only ones.

H and He are the obvious/simple answers because their valence shell can only hold two electrons. If you are in Grade 10 or 11, this may be the answer your teacher is looking for.

Basically, I don't think this question has only 2 answers. There are many elements that do not follow the octet rule. In fact, the octet rule is generally not correct, except for simple (1st and 2nd row) compounds

regards
Raghavendra
 

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BF3 and octet rule
« Reply #13 on: 23/03/2009 10:57:47 »

 

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