Exceptions to the "Octet Rule." There are two categories of exceptions to the octet rule among representative elements:
To help determine if the octet rule is followed recall Clark's Method (abbreviated) for determining bonding in covalent Lewis Structures:
- Add up all of the valence electrons in the structure (remember to add one electron for each negative charge, or subtract one for each positive charge)
- If e- = 6y + 2 where y = # atoms other than H, then octet rule is followed with single bonds only.
- If e- < 6y + 2 then probably have multiple bonding with the number of multiple bonds = /2 (remember a triple bond is 2 multiple bonds!). However, note the exceptions below with small atoms (H, Li, Be, and B).
- If e- > 6y + 2 then have an expanded valence shell. Note that if = 2, then pentavalent (10 electrons in the valence shell) , and if = 4, then hexavalent (12 electrons in the valence shell).
- If you can draw more than one structure, then chose the most symmetrical.
- If two or more structures are equally symmetrical, then you probably have resonance and should show all structures connected by double arrows.
Representative atoms with empty d-shells can also have what are sometimes referred to as expanded valence shells. In these cases the d-orbitals also participate in bonding enabling more bonds to be formed. Two additional electronic geometries are possible:
These two electron pair geometries can lead to six new molecular geometries in addition to another way to make a linear molecule:
Polarity: So now we can predict bonding and shape in representative group molecules (and thus most biomolecules), how about electron density and thus charge distribution? Need two bits of information:
|Molecule||Geometry||Structure||Electronegativities||Bond Dipoles||Molecular Dipole||Model|
||None: two dipoles are of equal magnitude, but opposite in direction and cancel.|
|None: four dipoles are symmetrically arranged to cancel each other out and give a spherically charged but non-polar ion.|
3. Determine Geometry:
- Electronic geometry = Trigonal bipyramidal:
- Since there are two lone pairs the trigonal bipyramidal electronic geometry give a T-Shape:
4. Determine Polarity:
- Is there a difference in the Electronegativity (EN) of the bonded atoms?
- ENCl = 3.0, ENF = 4.0
- EN = 1.0, there is a difference, and
- The Cl-F bonds are polar, with a small negative charge on each Fluorine.
- One of the bonds is not opposed (the molecule is NOT symmetrical), it is polar.
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© R A Paselk
Last modified 15 April 2013