Ppt23 (PS11)

1. Ppt23 (PS11) • (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 Ppt23(PS11)

2. 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.) Ppt23(PS11)

3. DH-H(bond energy of H-H) = +436 kJ/mol 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!) nuclei repel (energetically unfavorable) Ppt23(PS11)

4. Bond Energy NOTE: 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. • The energy required to break apart a mole of Cl2 molecules into Cl atoms is 243 kJ: Cl2(g)  2 Cl(g) ; DH = 243 kJ Thus, 243 kJ/mol is the bond energy of Cl-Cl Ppt23(PS11)

5. Ppt23(PS11)

6. When one atom is the same (e.g., H here), the bond lengths trend as the atomic radius of the other atom. Ppt23(PS11)

7. For bonds between the same two atoms, as bond order (1 = single; 2 = double, 3 = triple)* increases, bonds get stronger and shorter. Bond Energy (kJ/mol) 347 611 837 305 615 891 360 736 *As we shall soon see, non-integer bond orders (e.g., 4/3, 1.5) are also possible. 163 418 946 222 590 Ppt23(PS11)

8. Lewis Dot Structures-II • Resonance Structures/Forms • Formal Charge—Concept and Application • Special Case scenarios—Simple Organic Compounds Ppt23(PS11)

9. 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 Ppt23(PS11)

10. 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. Ppt23(PS11)

11. 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! Ppt23(PS11)

12. Figure 9.11 (b). Resonance Interpretation Ppt23(PS11)

13. Non-equivalent Resonance Structures • Which is better? • → To answer, assign “formal charges” (FCs) to each atom • → Then assess which structure has more “zeros”, fewer “charges” Ppt23(PS11)

14. 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! Ppt23(PS11)

15. How To Assign a FC to an Atom • Compare the number of electrons that an atom “wants” (to be neutral) to how many it “has” in the LDS**. 0 • If it has the same number, its FC = ___ -1 • If it has one more electron, its FC = ___ -2 • If it has two more, its FC = ___ +1 • If it has one electron fewer, its FC = ___ **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.”) Ppt23(PS11)

16. Revisit Earlier Slide—Examples:Assign FC’s to each atom in each LDS BETTER WORSE FC’s: 0 0 0 FC’s: +1 -1 0 > WORST BEST 2nd BEST (slightly) >> 0 +1 -1 -2 +1 +1 -1 +1 0 #v e-, if neutral 6 6 5 5 6 5 5 5 5 #v e-, in LDS 7 5 6 4 4 5 7 4 6 -1 +1 0 -2 +1 +1 0 -1 +1 FC Ppt23(PS11)

17. 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 a more electronegative atom is better Ppt23(PS11)

18. 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!! Ppt23(PS11)

19. “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! • (revisit this once electronegativity is defined) Ppt23(PS11)

20. 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) Ppt23(PS11)

21. Special Case Scenarios (simple organic compounds) See next slide → Ppt23(PS11)

22. 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 Ppt23(PS11)

23. Example 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”: • Because O “wants” two bonds (and two lone pairs) C on left: • Has 3 bonds; “wants” 4 bonds, no lone pairs • Add lone pair to N, move to next C • Make bond “down” because otherwise, N “up” would have 4 bonds when N “wants” 3 bonds and one lone pair Ppt23(PS11)