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• (Prior Ppt with procedure to generate one “good” LDS reflects Sections 9.5 and 9.7 in Tro. I will defer the discussion on electronegativity and bond polarity [9.6 in Tro] until later)

• Bond energies and Bond Lengths (9.10 in Tro)

• Resonance Structures/Forms (9.8 in Tro)

• Formal Charges—Concept and Application (9.8 and part of 9.9 [expanded octets] in Tro)

• Special Case scenarios—Simple Organic Compounds

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Practice (if time, need): Draw Lewis Structures For the Following

• Br3-

• O3

• CO2

• PO43-

• SO42-

• ClF3

• HCN

NOTE: Basic LDS’s are covered in Section 9.5 and 9.7 in Tro. We will be continuing with ideas in Sections 9.7 - 9.10 in this PowerPoint, although I will discuss 9.10 (bond energies &

lengths) first. I’ll discuss Section 9.6 (electronegativity and bond polarity) in the next PowerPoint presentation.)

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Like Fig. 10.6 in Tro. A bond is a state of lowered energy. A covalent bond is a shared pair of electrons—energy is lowered if electrons get to “see” (be attracted to) two nuclei instead of one. Bond energy is a measure of how hard it is to break a given bond. (E is released if a bond is made!)

DH-H (bond energy of H-H) = +436 kJ/mol

nuclei repel (energetically unfavorable)

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Bond Energy

• The energy required to break apart a mole of Cl2 molecules into Cl atoms is 243 kJ:

Cl2(g) 2 Cl(g) ; H = 243 kJ

Thus, 243 kJ/mol is the bond energy of Cl-ClNOTE: The larger the bond energy:the stronger the bond and the lower the bond “is” in potential energy.

Strong bonds don’t “have” a lot of energy! They’ve lowered themselves a lot in PE.

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When one atom is the same (e.g., H here), the bond lengths trend as the atomic radius of the other atom.

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For bonds between the same two atoms, as bond order (1 = single; 2 = double, 3 = triple)* increases,

bonds get stronger and shorter.

*As we shall soon see, non-integer bond orders (e.g., 4/3, 1.5) are also possible.

347

611

837

305

615

891

360

736

163

418

946

222

590

Bond Energy

(kJ/mol)

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Lewis Dot Structures-II

• Resonance Structures/Forms

• Formal Charge—Concept and Application

• Special Case scenarios—Simple Organic Compounds

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Resonance Structures (Forms, Hybrids)

• For two structures to be resonance structures (representing the same species):– The number and position of all atoms must be

identical (same skeleton structure)– The total number of valence electrons must be

identical (otherwise two different species are being represented!)

– At least one electron (or pair) has moved from one atom or bond to another atom or bond

• I.e., the only thing that differs is the relative placement of the valence electrons

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Equivalent Resonance Structures

• Example: O3 (next slide)

• Example: CO32- has three equivalent

resonance structures (draw them!):NOTE: The structures are analogous to those for NO3

- (shown on p. 380 of Tro), because nitrate ion and carbonate ion are isoelectronic! Just remember that carbonate has a -2 charge, not a -1 charge.

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Significance / Interpretation of Resonance Structures?

• Use to “patch up” an inadequate model!

• No single resonance structure is consistent with observations!– Experimentally determined structure for ozone

has two equivalent O-O “bonds”, each with a length intermediate between the average O-O bond and O=O bond!

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Figure 9.11 (b). Resonance Interpretation

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Non-equivalent Resonance Structures

• Which is better?

N N O N N ON N O

O C O O C O

→ To answer, assign “formal charges” (FCs) to each atom

→ Then assess which structure has more “zeros”, fewer “charges”

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Formal Charges—Concept• Each atom in an LDS can be assigned a formal

charge (FC)– Analogous to assigning each atom in a “formula unit” an

“oxidation number”, but the manner (and purpose) is different

• Did an atom “formally” gain or lose any electrons as a result of the bonding arrangement represented by the LDS?– If so, that is considered “nonideal” to some extent (takes

energy)

• NOTE: Atoms (in LDS’s) get formal charges; The LDS (as a whole) does not have a FC. The actual charge on an ion is not a FC!

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How To Assign a FC to an Atom• Compare the number of electrons that an atom

“wants” (i.e., to be neutral) to how many it “has” in the LDS**.

**How do you COUNT the electrons here? 1) Both electrons in a lone pair clearly belong to the

atom they’re “on”.2) ONE electron in a bond(ed pair) is given to each

atom involved in the bond. (“Cut the bond in half.”)

– If it has the same number, its FC = ___

– If it has one more electron, its FC = ___ – If it has two more, its FC = ___

– If it has one electron fewer, its FC = ___

0

-1

-2

+1

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Revisit Earlier Slide—Examples:Assign FC’s to each atom in each LDS

O C O O C O

N N O N N ON N O

FC’s: +1 FC’s: 0

#v e-, ifneutral#v e-, in LDS

5

5

FC 0

0

5

4

+1

+1

6

7

-1

-1

5

6

-1

-1

5

4

+1

+1

6

6

0

0

5

7

-2

-2

5

4

+1

+1

6

5

+1

+1

0 -1 0 0

WORSTBEST 2nd BEST (slightly) >>>

WORSE BETTER

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Which Resonance Structure is “Better” (Lower in Energy)?

• If ALL atoms have ZERO formal charge and octets, that is best! If not, assuming all atoms have octets, the:

• Ones with fewer non-zero formal charges are better

• Ones with smaller-magnitude non-zero formal charges is better

• If all above are same (tied), then the one with a negative FC on an atom farther to the right and up on the periodic table (the atom with a greater

electronegativity--later) is better.

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Formal Charges tend to be “bad” for “bad” skeleton structures!

• Try N2O with O in middle! (board)

• Try HCN with N in middle! (board)

• This provides a rationalization for the “lower and to the left” goes in the middle” rule about skeleton structures!– Some instructors never even give that rule!

They make you figure out which is best by doing all possible ones and picking the best one based on formal charges!!

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“Formal Charge” idea rationalizes the preferred LDS for BF3 (exception)

• See board– My procedure would yield a B-F double bond

(B=F) to get “eight electrons around the center”.

– But…this makes the FC on F a +1. “Bad”!• Very hard to pull an electron away from F!

– High effective nuclear charge, right?

• (revisit this once electronegativity is defined)

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Formal Charges vs. Octet?

• Sometimes the Resonance Structure with the Best Formal Charges has atoms with more than 8 electrons around it.– i.e., sometimes, to minimize FCs, you need to “break”

the octet rule (see examples, below)

• In such cases, I would never ask you “Which is best?” without clarifying– With respect to octet rule?– With respect to formal charges?

• Examples: SO2, SO42- (on board)

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Special Case Scenarios (simple organic compounds)

• See next slide →

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Molecules with C, N, O, and “H” (hydrogen or halogen)

• C, N, and O are in the 2nd row: don’t go over 8 e-’s• In organic compounds (C & H present), these atoms

usually follow the “simple” patterns below

[NOTE: these patterns will lead to both “octets” (duet for

H) and formal charges of ZERO for all atoms!]

• C has 4 v e-’s: forms 4 bonds and has no lone pairs

• N has 5 v e-’s: forms 3 bonds and has one lone pair

• O has 6 v e-’s: forms 2 bonds and has 2 lone pairs

• Halogen, 7 v e-’s: forms 1 bond, has 3 lone pairs

• H has 1 ve: forms 1 bond, no lone pairs

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Example

C

N C

C

NC

HH

C

N H

C

O

OH

Cl

H

H

H

H

Create an LDS from the skeleton structure below by adding lone pairs AND POSSIBLY MAKING DOUBLE OR TRIPLE BONDS so that the octet rule AND the simple patterns for C, N, and O are “satisfied”:

C on left:• Has 3 bonds; “wants”

4 bonds, no lone pairs• Make bond “down” because otherwise,

N “up” would have 4 bonds when N “wants” 3 bonds and one lone pair

• Add lone pair to N, move to next C

• Because O “wants” two bonds (and two lone pairs)

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