7.8 NOTES Exceptions to the Octet Rule

B. Resonance structures –Certain molecules that are composed of single AND double bonds can be written in two different ways - For example - O3. - If one is chosen over another it presents the problem of determining bond length; the single bond is longer than the double bond, but it has been proven both bonds in ozone have the same length - Answer – use both structures to represent the molecule; known as resonance structures

- RS – 1 of 2 or more Lewis structures for a single molecule that cannot be described fully with only 1 Lewis structure - Resonance – the use of 2 or more Lewis structures to represent a particular molecule;

C. Exceptions to the octet rule 1. Odd electron species, e.g. NO or NO2 - Small number of molecules will have an odd number of electrons and therefore cannot form an octet - ALL OF SERIES 2 CAN NEVER HAVE MORE THAN 8 ELECTRONS

2. Fewer than 8 electrons. BH3 is the best example. Coordinate covalent bond: A covalent bond in which one of the atoms donates both electrons; some compounds have fewer than 8 electrons and tend to be reactive BH3 bonds with NH3, with NH3 donating both electrons creating a coordinate covalent bond atoms or ions with lone pairs often form coordinate bonds; B, H, and halogens can never make a double bond b/c lack the # of orbitals Examples: BeH2 H – Be – H; Be configuration 1s22s2; BX3 where X is a halogen

3. Expanded octet. Larger atoms (series 3 and beyond) can hold more than 8 e-. P, S, and As commonly have 10 or 12 e- around them. Elements in (and beyond) the 3rd series have d orbital electrons that can be used in bonding as well as s and p SF6 is an example  each of sulfur’s valence electrons will form a covalent bond with a fluorine atom giving S twelve electrons