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Unformatted text preview: correct total charge on the molecule or molecular ion. correct What is the formal charge on each carbon atom
2 4 in ethylene (C H )? in C ≡ group no. − no. of lone pair electrons − ½ (no. of electrons in bonding pairs) (no. 12/04/09 Zumdahl Chapter 13 48 What is the formal charge on each hydrogen atom
2 4 in ethylene (C H )? in C ≡ group no. − no. of lone pair electrons − ½ (no. of electrons in bonding pairs) (no. As should be the case for the neutral molecule C2H4, 12/04/09 Zumdahl Chapter 13 49 1. Determine the formal charge (C) on each atom, and write it next to that atom. Check that the formal charges add to give a correct total charge on the molecule or molecular ion. What are the formal charges on the atoms What in carbon monoxide (CO)? in C ≡ group no. − no. of lone pair electrons − ½ (no. of electrons in bonding pairs) (no. C O As should be the case for the neutral molecule CO
12/04/09 Zumdahl Chapter 13 50 Resonance Structures
There are cases where more than one Lewis structure can be There .. .. drawn for a molecule: drawn
Equivalent Structures OOO Typical O−O single bond length: 1.49 Å single 1.49 Typical O=O double bond length: 1.21 Å Typical 1.21 In O3 the experimental bond lengths are the same: 1.28 Å 1.28 The actual structure can be described as a combination of the two Lewis structures, i.e., a
.. .. OOO
Zumdahl Chapter 13 51 More Examples of Resonance Structures
Carbonate anion, CO32−
O C O O O -2− 2 O C O -2 2− O C O O -2 2− Thiocyanate anion, NCS−
N C S -− 1 N C S -− 1 N2O (nitrous oxide, laughing gas)
N N O N N O N N O 12/04/09 Zumdahl Chapter 13 52 Favored Resonance Structures
When comparing resonance structures for the same molecule, When usually those with the contribute more to the overall resonance hybrid. When formal charges are necessary, resonance structures that have and positive charges on the less electronegative elements typically contribute more to the overall resonance hybrid. contribute Examples: N2O and NCS Examples: NCS
12/04/09 Zumdahl Chapter 13 53 Breakdown of the Octet Rule
(free radicals) such as NO: such NO N # of valence e− = 5 (N) + 6 (O) = 11 11
Lewis structure does not satisfy the octet rule but the molecule is Lewis stable, although somewhat reactive. O Case 2:
F B F F (mainly Be, B, and All compounds): mainly Be and A (A) F -1 B F F +1 (B) Even though structure (B) obeys the octet rule, structure (A) is favored Even (B) (A) (Formal charges are lower & experimental evidence shows that there is no double bond in BF3 ). double
12/04/09 Zumdahl Chapter 13 54 Breakdown of the Octet Rule (con’t.)
Case 3: Third and higher period elements can exhibit bonding where an Third
− octet on the central atom is exceeded -- can expand up 12 e ! can
6 F F 4 F F S F F SF F F S F 34 total valence e 8 e− in bonds 24 e− in lone pairs 2 e− left over SF − Rule for Lewis structures: Rule If electrons remain after satisfying the octet rule, 12/04/09 Zumdahl central add lone pairs to the Chapter 13 atom!
55 The Shapes of Molecules: The VSEPR Theory VSEPR ≡ Valence Shell VSEPR Electron-Pair Repulsion Theory Electron-Pair 12/04/09 Zumdahl Cha...
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