dolimitless
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Exceptions to the Octet Rule
I know that H, He, Li, Be, and B can form less than 8 electrons, but I don't understand the explanation for how elements in period 3 or greater can
obtain more than 8 electrons in their valence shell. I know it is something to do with exciting atoms from the p to d subshell. I found this
explanation online, but I cannot make sense of it. Can someone explain more clearly? This forum is great for that!!:
"Technically, elements in periods 3-7 should be able to exceed the octet, as they all have the d orbital available for an expanded octet. With
that being said, the ability to exceed the octet is all about the energy that an atom has. At ground state, most elements follow the octet rule. But
as an atom absorbs energy, one or more of the valence electrons will move from s/p orbitals to the d orbitals (doing the reverse of Hund's rule),
which results in more bonding sites becoming available. Sulfur and Phosphorus are good examples of elements that commonly exceed the octet rule.
Sulfur is 1s2 2s2 2p6 3s2 3p4. 3s2 and 3p4 make up the valence shell. According to Hund's rule, only 2 of the orbitals in the p sublevel are available
for bonding. However, add a little energy, and one of the s and one of the p orbitals get promoted to their own orbitals in the d sublevel. So now
there are 6 orbitals available for bonding (instead of the normal max of 4) because the s orbital is half-filled, each of p's 3 orbitals is
half-filled, and d has 2 half-filled orbitals. The energy difference between p and d is not that great...they are very close to each other in the
electron cloud."
Can someone explain more clearly? The users on this forum are great for explaining, thanks for your help!
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woelen
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Elements in period 1 only have an s subshell in main shell number 1.
Elements in period 2 only have an s subshell and a p subshell in main shell number 2.
Elements in period 3 have an s subshell, a p subshell and a d subshell in shell number 3.
All higher elements also have these three shells, plus additional ones.
If you look at this, then you can see that for period 1 and period 2 elements there simply is no possibility to extend their bonding abilities beyond
1orbital for period 1 and 4 orbitals for period 2. There simply are no other subshells in the main shell, which is used for bonding by elements in
these periods. A period 2 element does not use sub shells from period 3, because that would be energetically very unfavorable. From period 3 on you
see that the main shell, used for bonding has a d subshell as well and now there are more possibilities of forming bonds. This is possible, because
the energy levels for promoting to these d subshells is much less than going to a higher main shell level.
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kmno4
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Quote: Originally posted by woelen  |
(...)
If you look at this, then you can see that for period 1 and period 2 elements there simply is no possibility to extend their bonding abilities beyond
1orbital for period 1 and 4 orbitals for period 2.
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It is not so simple as it seems.
Read about hypervalency/multicenter bonds.
BTW. there is a nice article (ACS, ja0438011) about pentacoordinate
carbon and boron compounds.
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woelen
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Multicenter bonds actually perfectly adhere to the octet rule. Best example is B2H6.
Structure is H2BHHBH2. The left and right 4 H-atoms share a single electron with the boron.
The boron-atoms also share a single electron with each of the left and right H-atoms. The s-orbitals of the H-atoms are filled and these H-atoms are
happy.
The middle H-atoms share their electron with two boron atoms and one of the boron atoms also shares its third electron in the second main shell. The
same construction is true for the other middle H-atom and this also shares its electron with two boron-atoms, while one of the boron atoms also shares
its electron.
So, the middle atoms and both boron atoms share two electrons, while three atoms are involved. This is a so-called 2 electron 3 center bond (the most
common bond is 2 electron 2 center). But with this structure, each of the 6 H-atoms shares 2 electrons and each boron atom shares 8 electrons (4 from
two 2 electron 2 center bonds with the left and right H-atoms and four from two 2 electron 3 center bonds).
So, this muticenter type of bonding does not violate the octet rule, it even shows that the octet rule is strong for period 2 elements.
The pentacoordinate carbon compounds most likely are very unstable and not the normal mode of bonding, while the 2 electron 3 center bonds for boron
are quite stable, so much that simple BH3 cannot exist, it immediately dimerizes to B2H6.
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JohnWW
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An interesting possibility where it was thought the octet rule for 2nd-period elements might be broken, and hence quantum-mechanical theory violated,
was the hypothetical existence of covalent nitrogen pentafluoride, NF5, as a trigonal-bipyramidal molecule like PF5, with 10 electrons in 2s and 2p
orbitals on the N atom and possibly utilizing the vacant 3s orbital. On the basis of electronegativity difference between N and F this is a
possibility. However, as I stated last year on http://www.sciencemadness.org/talk/viewthread.php?action=pri... , such a molecule has not been synthesized.
By the way, the NF4+ cation, isolectronic with CF4 and BF4-, was first synthesized in 1966, and has been found to be very stable in salts of strong
acids such as NF4ClO4, NF4BF4, NF4NO3, NF4PF6, and NF4SbF6. It, and the ClF6+ cation, can be made by adding F+ to NF3 and ClF5 by their reactions with
the salt [KrF+][SbF6-], which is the only compound of Kr stable at room temperature. The cation OF3+ should theoretically be accessible by the same
method, but has still not been synthesized; see https://www.sciencemadness.org/whisper/viewthread.php?action... and http://linkinghub.elsevier.com/retrieve/pii/S002211399900139... (someone please post this complete article) .
BUT, the ionic pentafluoride, NF4+F-, has still not been isolated, let alone covalent NF5. By the 1992 article abstract http://pubs.acs.org/doi/abs/10.1021/ja00051a027 (someone please post the full article), it is expected to be exothermically unstable with respect
to decomposition to NF3 and F2 at -142ºC, even if it could be made.
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kmno4
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My citation of woelen's post is not too correct, sorry.
I meant that including d orbitals into bondings is not so obvious for early elements of main groups at all. For 2nd period it is rather excluded but
multicenter orbitals are not excluded (for example in NO4(3-) ion) . In this context I recommend further reading (for the concerned).
BTW. I saw orbital diagrams for SO3 or something similar (for its spectra explanation): one with d obitals contribution and another without these
orbitals. Interpretation belongs to school.... 
[Edited on 15-6-2009 by kmno4]
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