Lewis Structures
Times when an atom doesn't have an octet...
(June 30, early AM)
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The real ambition of the atoms in molecules is to maximize their bonding interactions.  There are limitations.  One simply cannot keep drawing lines between atoms without justification.

Limitation 1: each bond represents two electrons, there can only be so many bonds based on the number of valence electrons present.

Limitation 2: each atom has only a certain number of valence orbitals which it can use while making bonds.  Hydrogen only has a 1s orbital available so it's total electron count is limited to 2 ( a single bond).  The second period elements (with only 2s + 2p available) are limited to total electron counts of 8.

Limitation 3: Geometric constraints arise because the atomic orbitals on atoms have certain directional character (with the exception of "s" orbitals which are spherical).  This allows them to bond effectively in only certain ways.  This limitation will be expounded upon later, but suffice it to say that two carbon atoms bonded in the gas phase (C2) cannot exhibit a bond order of 4 because all three 2p orbitals cannot localize electron density between the nuclei simultaneously.

Key Points:

*Atoms in molecules want to make bonds (within reason), not necessarily attain an octet of electrons.

*The number of available orbitals and electrons gives rise to an "octet" rule for 2nd period elements which maximize their bonding interactions when the attain a total electron count of 8.


Exceptions to the "Octet Rule"

We have already seen and are quite comfortable with the fact that hydrogen cannot handle anything but a total electron count of 2.  This should open your eyes to the reality underlying Lewis Structures!  This is the first example of a violation to the so-called octet rule.

What about a substance like BH3?  The best Lewis structure is depicted below.
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Each hydrogen exhibits it's maximum total electron count (2) and not surprisingly it's maximum number of bonds.  The boron atom in BH3, on the other hand, has only 6 total electrons.  This apparent problem is not a problem when one considers the principles discussed above AND the fact that the real "rule" is that boron cannot EXCEED an octet of electrons.

Consider BF3 as well.
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I consider the best Lewis Structure to be the one shown on the left (I), while few would argue that the one on the right (II) is better.  The presence of lone pair electrons on each F atom potentially allows additional bonding interactions.  The problem with II is the formal charges which imply that electronegative fluorine is losing the electron density tug-of-war with boron!  (The electronegativity difference between these two elements is similar to the difference between Na and Cl, a strong electrolyte!)

Another case where the "octet rule" must be violated are reactive species with an odd number of valence electrons.  A perfect example is nitrogen monoxide (nitric oxide), NO.  This species has 11 valence electrons.  There is absolutely NO way to have all the electrons pair up and one of the atoms must have an odd total electron count as well.  The best Lewis structure maximizes the number of bonds but does not violate the true octet rule: neither 2nd period atom exceeds an octet!
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The last type of "apparent" exception to the octet rule occurs when an element of the 3rd period or higher is the central atom in a molecule or molecular ion AND it is bonded to electronegative substituents such as O, F, Cl, Br.  Examples: XeF4, SF6, PCl5, [SeBr6]2-, IO4-, ClO4- etc...

For the first 4 examples (XeF4, SF6, PCl5, [SeBr6]2-) the Lewis structures are easily drawn since terminal halogens and fluorine always contain only single bonds and three lone pairs in the best structure.  Let's consider the periodate ion, IO4-.  Several Lewis structures are drawn below.
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Several Lewis Structures for Periodate
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Structure I looks reasonable because each atom posseses an octet of electrons.  The large formal charge (QF) on the central iodine atom (+3) is energetically unfavorable as are the fewer number of bonds.  Structure I is actually the highest energy structure shown.

Structures II and III reduce the formal charge on the central I atom to more reasonable levels, III being realistic energetically since the iodine atom only bears a QF of +1.

The best structure shown is IV, which posseses a greater number of bonds, as well as the minimum formal charges.  The placement of a negative formal charge on oxygen is entirely realistic.  Note that the central iodine atom exhibits a total electron count of 14 in structure IV (the best structure) but has the supposedly magical 8 in structure I (the worst).

Below is shown one final structure (V) for IO4- in which an additional bond between oxygen and iodine is drawn.  This representation is entirely acceptible but is higher in energy when compared to IV because the negative charge is borne by the less electronegative central iodine atom instead of the more electronegative oxygen atom.
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Click here to review procedures for drawing Lewis Structures etc...
Click here to move on to the next section of notes: resonance