Exceptions to the Octet Rule

1. Exceptions to the Octet Rule

2. 1. Electron-odd Species: • Covalent compounds that have an odd number of total valence electrons • Results in an unpaired electron in the Lewis structure • i.e.: NO2

3. 1. Electron-odd Species: • In this case, we refer to these compounds as paramagnetic • refers to a strong attraction to an external magnetic field due to an unpaired electron (s) • considers electrons as spinning electric charges that create a magnetic field • paired electrons have spins in opposite directions and the magnetic field cancels out as a result there is little to no attraction to an external magnetic field • compounds consisting of electrons that are all paired up are known as diamagnetic compounds

4. 2. Incomplete Octets: • Results in the deficiency of an electron pair • The only example I have found is the boron atom of the compound boron trifluoride • i.e.: BF3

5. 2. Incomplete Octets: • No double bonds are formed for this molecule • inspection showed that only single bonds were present • high electronegativity difference between B and F suggests potential ionic character • readily reacts with NH3 to make the compound BF3 NH3 completing the octet • i.e.:

6. 3. Expanded Valence Shell: • Results when the central atom has more than 8 electrons in its valence shell • Typically occur when a non-metal central atom in the 3rd energy level or higher is bonded to highly electronegative terminal atoms

7. 3. Expanded Valence Shell: • i.e.: PCl3 & PCl5

8. 3. Expanded Valence Shell: • i.e.: SF6

9. 3. Expanded Valence Shell: • Where do these “extra” electrons go? • rationalized by the idea that once the s and p orbitals are filled for the central atom (8 electrons), extra electrons are then filled in the d orbital • it is believed that the energy difference between the p and d orbitals isn’t very large, so the valence shell expansion seems feasible