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Formal Charge

Formal Charge. Edward A. Mottel Department of Chemistry Rose-Hulman Institute of Technology. H. •. •. H. O. H. C. •. •. H. Formal Charge. Assume bonding electrons belong 1/2 to each bonding atom. The formal charge is the net charge on each atom. e – shared. 2. H. •. •. H. O.

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Formal Charge

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

  2. H • • H O H C • • H Formal Charge Assume bonding electrons belong 1/2 to each bonding atom. The formal charge is the net charge on each atom.

  3. e–shared 2 H • • H O H C • • H Formal Charge unshared electrons formal charge - = e–donated - formal charge of oxygen 6 valence electrons 4/2 shared electrons 4 unshared electrons = - -

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

  5. H H • • • • • • • • • H C H H O H C O • • • • • • • • • C: 4 - 6/2 - 2 = -1 C: 4 - 4/2 - 4 = -2 H H H: 1 - 2/2 - 0 = 0 H: 1 - 2/2 - 0 = 0 O: 6 - 6/2 - 2 = +1 O: 6 - 8/2 - 0 = +2 Incorrect Diagrams for Methanol 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. Validity Check The sum of the formal charges always equals the charge on the molecule or ion.

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

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

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

  9. Formal Charges • What are the formal charges on the atoms in • hydroxide ion • hydronium ion (oxonium ion)

  10. O – O O Cl Chlorate Ion Skeleton diagram usually the unique atom is in the center two possibilities Cl-O-O-O – chain structures are uncommon correct structure

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

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

  13. Pair ActivityFormal 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.

  14. Pair ActivityFormal 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, CH3COOH • acetate ion, CH3COO–

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

  16. Formal Charges • What are the formal charges on the atoms in • ozone • sulfite ion • sulfur trioxide

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

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

  19. • • • • • • • • • • O O O • • • • • • • • • • • • • • • • • • • • • • • • C O • • C O C O • • • • • • • • • • • • • • • • • • • • • • • • • O O • O • • • • • • • • • • Resonance 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.

  20. • • • • • • • • • • O O O • • • • • • • • • • • • • • • • • • • • • • • • C O • • C O C O • • • • • • • • • • • • • • • • • • • • • • • • • O O • O • • • • • • • • • • Resonance What is the carbon-oxygen bond order? (1+1+2) / 3 = 1.33

  21. • • • • • • • • • • O O O • • • • • • • • • • • • • • • • • • • • • • • • C O • • C O C O • • • • • • • • • • • • • • • • • • • • • • • • • O O • O • • • • • • • • • • Resonance Where is the negative charge located in each contributing structure? On the oxygen atoms with a single bond.

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

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

  24. - - • • • • • • • • - • • • • • • • • • • • • • • • • N N N S C S C S • • • • • • • • • • • • • • • • • • • • • • • • 0 0 -1 +1 0 -2 -1 0 0 A “double headed” arrow is the symbol for resonance. Thiocyanate Ion Possible Lewis Dot structures - C What carbon-sulfur and carbon-nitrogen bond orders are predicted for this ion? Formal charges favor which of these structures? C-S: 1.5; C-N: 2.5 (nitrogen is slightly more electronegative than sulfur: 3.1, 2.4)

  25. 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.

  26. • • 2- 2- • • O O • • • • • • • • • • • • • O O O O S S • • • • • • • • • • • • • O O S: 6 - 8/2 - 0 = +2 • • • • O: 6 - 2/2 - 6 = -1 O: 6 - 2/2 - 6 = -1 O: 6 - 2/2 - 6 = -1 O: 6 - 2/2 - 6 = -1 total charge = -2 Sulfate Ion Skeleton diagram Lewis Dot diagram S 6 valence e– O 24 valence e– charge 2 e– total 32 valence e– 16 e– pairs Is sulfate ion a good oxidizing agent? Is it explosive?

  27. Rationalizing the Inertness of theSulfate 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.

  28. 2- • • O • • • • • • • • O O S • • • • • • • • S: 6 - 12/2 - 0 = 0 O • • O: 6 - 2/2 - 6 = -1 O: 6 - 4/2 - 4 = 0 O: 6 - 2/2 - 6 = -1 O: 6 - 4/2 - 4 = 0 total charge = -2 Formal Charges of AtomsSulfate Ion Where are the two negative charges located? How many different resonance configurations can be drawn?

  29. • • • • • • 2- 2- 2- • • • • • • O O O • • • • • • • • • • • • • • • • • • • • O O S O O S O O S • • • • • • • • • • • • • • • • • • • • O O O • • • • • • • • 2- 2- 2- • • • • • • O O O • • • • • • • • • • • • • • • • • • • • • • O O O O S O S O S • • • • • • • • • • • • • • • • • • • • • • O O O • • • • • • • • • • Resonance Forms of Sulfate Ion What is the sulfur and oxygen bond order? Why is sulfate ion relatively inert? What is the average formal charge on each atom?

  30. H H • • • • H H H H C C • • • • • • • • • • • • • • • • • • C C • • C C • • • • • • • • • • • • C C C C • • • • • • • • • • • • • • • • • • H H • C C • H H • • • • C 24 valence e– H H H 6 valence e– It is assumed that there are enough hydrogen atoms at each corner to fulfill carbon’s valence. Benzene, C6H6 Shorthand notation Even shorter notation Cyclic structure total 30 valence e– In this structure, what does the circle represent? What is the carbon-carbon bond order?

  31. H H H – – What is the shorthand notation for the cyclopentadienide ion? Cyclopentadiene Cyclopentadiene reacts with hydroxide ion to make the cyclopentadienide ion. + OH– + H2O One of several resonance structures What is the carbon-carbon bond order in C5H5–? What is the formula of cyclopentadiene?

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