Drawing Lewis Dot Diagrams & Resonance Mrs. Paparella SUPA CHEMISTRY December 2013

Lewis Dot Structure Rules: Treat ions separately (e.g. NH4Cl) Count only valence electrons Assemble bonding framework (skeleton) Fill up non-bonding electrons on outer atoms inner atoms

To do Lewis Structures: Must be able to recognize polyatomic ions Must be able to identify valence electrons Must be able to construct Bond framework (skeleton) Periodic Table : Column numbers!

Hints on Lewis Dot Structures Octet “rule” is the most useful guideline. Carbon forms 4 bonds. Hydrogen typically forms one bond to other atoms. When multiple bonds are forming, they are usually between C, N, O or S. Nonmetals can form single, double, and triple bonds, but not quadruple bonds. Always account for single bonds and lone pairs before forming multiple bonds. Look for resonance structures.

5+(3*7)=26 e- PCl3 1. Count the total number of valence e- P has 5 and each Cl has 7 for a total of 26 e- to be placed 3. Establish octet around each Cl. Bonding Pairs 4. 26e- - 24e- so far = 2 e- left; place above P. 2. Identify central atom and then place a line between it and each additional atom (skeleton).The line counts as 2 e- Lone Pairs (a.k.a. nonbonding electrons)

Try Some Examples: CH3CH2NH2 Cl2CO Ozone (O3) NO2 vs. N2O SPEND LOTS OF TIME PRACTICING…

Covalent Bonding Multiple Bonds It is possible for more than one pair of electrons to be shared between two atoms (multiple bonds): One shared pair of electrons = single bond (e.g. H2); Two shared pairs of electrons = double bond (e.g. O2); Three shared pairs of electrons = triple bond (e.g. N2). Generally, bond distances shorten with multiple bonding. Octet in each case

Odd Number of valence Electrons… Won’t have octet NO Number of valence electrons = 11; N has 5 and O has 6 Resonance occurs when more than one valid Lewis structure can be written for a particular molecule (i.e. rearrange electrons) NO2 Number of valence electrons = 17 Molecules and atoms which are neutral (contain no formal charge) and with an unpaired electron are called Radicals O2

Beyond the Octet Elements in the 3rd period or higher can have more than an octet if needed. Atoms of these elements have valence d orbitals, which allow them to accommodate more than eight electrons.

More than an Octet… Elements from the 3rd period and beyond, have ns, np and unfilled nd orbitals which can be used in bonding P : (Ne) 3s2 3p3 3d0 Number of valence electrons = 5 + (5 x 7) = 40 PCl5 SF4 S : (Ne) 3s2 3p4 3d0 Number of valence electrons = 6 + (4 x 7) = 34 The Larger the central atom, the more atoms you can bond to it – usually small atoms such as F, Cl and O allow central atoms such as P and S to expand their valency.

Resources and Practice See text book p 269-279