Formal Charge Edward A. Mottel Department of Chemistry Rose-Hulman Institute of Technology

6/27/2015 Formal Charge The formal charge is the net charge on each atom. Assume bonding electrons belong 1/2 to each bonding atom. H C H HO H

6/27/2015 Formal Charge formal charge unshared electrons = e – donated - e – shared 2 - formal charge of oxygen 6 valence electrons = 4/2 shared electrons -- 4 unshared electrons H C H HO H

6/27/2015 C O H H H H 4 - 8/2 - 0 = /2 - 0 = /2 - 4 = 0 Formal Charges of Atoms Methanol Formal charge = valence electrons - electrons shared/2 - electrons not shared C: 4 - 8/2 - 0 = 0 H: 1 - 2/2 - 0 = 0 O: 6 - 4/2 - 4 = 0 Atoms with low formal charges generally indicate the correct bonding pattern of chemically stable molecules.

6/27/2015 Incorrect Diagrams for Methanol O H H H CH C: 4 - 6/2 - 2 = -1 H: 1 - 2/2 - 0 = 0 O: 6 - 6/2 - 2 = +1 Validity Check The sum of the formal charges always equals the charge on the molecule or ion. H C HO H H C: 4 - 4/2 - 4 = -2 H: 1 - 2/2 - 0 = 0 O: 6 - 8/2 - 0 = +2 What are the formal charges for each atom in the following structures? Validity Check The sum of the formal charges always equals the charge on the molecule or ion.

6/27/2015 Incorrect Diagrams for Methanol O H H H CHH C HO H H C: 4 - 4/2 - 4 = -2 H: 1 - 2/2 - 0 = 0 O: 6 - 8/2 - 0 = +2 C: 4 - 6/2 - 2 = -1 H: 1 - 2/2 - 0 = 0 O: 6 - 6/2 - 2 = +1 O H H H CH If either of these structures was stable, which one would more likely be stable?

6/27/2015 CN Cyanide Ion N 5 valence e – total 10 valence e – C 4 valence e – 5 e – pairs Skeleton diagram What are the formal charges for each atom in this ion? CN charge 1 e – Convert to a Lewis Dot diagram C: 4 - 6/2 - 2 = -1 N: 5 - 6/2 - 2 = 0 total charge = -1 Lewis Dot diagram

6/27/2015 Cyanide Ion C: 4 - 6/2 - 2 = -1 N: 5 - 6/2 - 2 = 0 total charge = -1 CN Which “end” of cyanide ion is negative? CN Cyanide ion reacts with iron(III) ion to make the hexacyanoferrate(III) ion. Does the iron(III) ion bond to carbon or nitrogen?

6/27/2015 Hexacyanoferrate(III) ion Fe C N C N C N C N C N N C 3- Carbon is the negative end of the cyanide ion.

6/27/2015 Ammonium Ion Nitrogen usually forms 3 bonds NHH H H + N 5 valence e – total 8 valence e – H 4 valence e – charge -1 e – Skeleton diagramLewis Dot diagram 4 e – pairs N HH H H + H: 1 - 2/2 - 0 = 0 N: 5 - 8/2 - 0 = +1 total charge = +1 Which atoms have a non-zero formal charge? In an ion, some atom must have a non-zero formal charge. Why?

6/27/2015 Formal Charges  What are the formal charges on the atoms in hydroxide ion hydronium ion (oxonium ion)

6/27/2015 Chlorate Ion Cl-O-O-O – – O O O Cl Skeleton diagram two possibilities usually the unique atom is in the center chain structures are uncommon correct structure

6/27/2015 Chlorate Ion O 18 valence e – total 26 valence e – charge 1 e – Cl 7 valence e – Only 6 electrons shown (3 pairs) O O O Cl Skeleton diagramLewis Dot diagram 13 e – pairs O O O Cl Cl: 7 - 6/2 - 2 = +2 O: 6 - 2/2 - 6 = -1 total charge = -1 O: 6 - 2/2 - 6 = -1 Why is chlorate ion a good oxidizing agent? Complete the Lewis Dot diagram

6/27/2015 Perchlorate Ion O 24 valence e – total 32 valence e – charge 1 e – Cl 7 valence e – Only 8 electrons shown (4 pairs) O OCl O O Skeleton diagramLewis Dot diagram 16 e – pairs O O O O Cl Cl: 7 - 8/2 - 0 = +3 O: 6 - 2/2 - 6 = -1 total charge = -1 O: 6 - 2/2 - 6 = -1 Why are organic perchlorate salts potentially explosive?

6/27/2015 Pair Activity Formal Charges  Develop a procedure to determine the formal charges on a molecule or ion. Initially, one person should be responsible for writing all the information. The other person should be responsible for saying what should be written.

6/27/2015 Pair Activity Formal Charges  On a card or a piece of paper write each sequential step necessary to determine the formal charges. number each step. amend and update the list.  Determine the formal charges on acetic acid, CH 3 COOH acetate ion, CH 3 COO –

6/27/2015 Acetic Acid and Acetate Ion O H C H H H C O O O C H H H C O C H H H C O C: 4 - 8/2 - 0 = 0 total charge = 0 H: 1 - 2/2 - 0 = 0 O: 6 - 4/2 - 4 = 0 C: 4 - 8/2 - 0 = 0 total charge = -1 H: 1 - 2/2 - 0 = 0 O: 6 - 2/2 - 6 = -1 O: 6 - 4/2 - 4 = 0

6/27/2015 Formal Charges  What are the formal charges on the atoms in ozone sulfite ion sulfur trioxide

6/27/2015 “Connection” diagram Convert to a Lewis Dot diagram Carbonate Ion charge 2 e – O 18 valence e – total 24 valence e – C 4 valence e – O O O C 2- O O O C 2- Lewis Dot diagram 12 e – pairs Which oxygen atom has the double bond?

Resonance Edward A. Mottel Department of Chemistry Rose-Hulman Institute of Technology

6/27/2015 Resonance C O O O C O O O C O O O For some skeleton and dot structures there is not an unique, correct answer. Carbonate ion has three resonant configurations. Individually, none of these structures represents the actual structure. But the average of these structures is a good representation of the ion.

6/27/2015 Resonance C O O O C O O O C O O O What is the carbon-oxygen bond order? (1+1+2) / 3 = 1.33

6/27/2015 Resonance C O O O C O O O C O O O Where is the negative charge located in each contributing structure? On the oxygen atoms with a single bond.

6/27/2015 Resonance C O O O C O O O C O O O What is the average formal charge on each atom in the carbonate ion? carbon: 0; oxygen: -2/3

6/27/2015 Thiocyanate Ion Only 8 electrons shown (4 pairs) total 16 valence e – charge 1 e – S 6 valence e – C 4 valence e – N 5 valence e – Skeleton diagrams N SC N SC

6/27/2015 N S C - N S C - N S Thiocyanate Ion C - Formal charges favor which of these structures? Possible Lewis Dot structures A “double headed” arrow is the symbol for resonance. What carbon-sulfur and carbon-nitrogen bond orders are predicted for this ion? C-S: 1.5; C-N: 2.5 (nitrogen is slightly more electronegative than sulfur: 3.1, 2.4)

6/27/2015 Resonance Structures  Factors to consider when picking the best resonance structures: Zero or low formal charges are the best. Formal charges are consistent with the electronegativity of the elements involved. Alternating positive and negative formal charges are better than highly polarized patterns.

6/27/2015 Sulfate Ion charge 2 e – O 24 valence e – total 32 valence e – S 6 valence e – O OS O 2- O Skeleton diagram Lewis Dot diagram 16 e – pairs O O O O S 2- Is sulfate ion a good oxidizing agent? Is it explosive? S: 6 - 8/2 - 0 = +2 O: 6 - 2/2 - 6 = -1 total charge = -2 O: 6 - 2/2 - 6 = -1

6/27/2015 Rationalizing the Inertness of the Sulfate Ion  The octet rule (8 electrons around an atom) is based on use of one s-orbital and three p- orbitals.  If d-orbitals are included, the octet rule is broken. Expanding the valence results in illegal Lewis Dot structures, but may explain the inert chemistry of the sulfate ion.

6/27/2015 Formal Charges of Atoms Sulfate Ion S: /2 - 0 = 0 O: 6 - 2/2 - 6 = -1 total charge = -2 O: 6 - 4/2 - 4 = 0 O: 6 - 2/2 - 6 = -1 O: 6 - 4/2 - 4 = 0 Where are the two negative charges located? O O O O S 2- How many different resonance configurations can be drawn?

6/27/2015 Resonance Forms of Sulfate Ion O O O O S 2- O O O O S 2- O O O O S 2- O O O O S 2- O O O O S 2- O O O O S 2- What is the sulfur and oxygen bond order? What is the average formal charge on each atom?Why is sulfate ion relatively inert?

6/27/2015 C H C C C H C C H H H H C H C C C H C C H H H H Benzene, C 6 H 6 total 30 valence e – H 6 valence e – C 24 valence e – Cyclic structure Shorthand notation It is assumed that there are enough hydrogen atoms at each corner to fulfill carbon’s valence. Even shorter notation In this structure, what does the circle represent?What is the carbon-carbon bond order?

Cyclopentadiene Cyclopentadiene reacts with hydroxide ion to make the cyclopentadienide ion. HH + OH – H – + H 2 O One of several resonance structures What is the formula of cyclopentadiene?What is the carbon-carbon bond order in C 5 H 5 – ? – What is the shorthand notation for the cyclopentadienide ion?

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